The hacks are pretty gnarly: you have to dump the game's ROM to your computer, modify it, and then reflash the console.
You are legally allowed to format shift your own games, but bypassing "effective means of access control" on copyrighted works is a potential violation of section 1201 of the DMCA, which provides for both criminal and civil penalties.
The fuggly nature of the jailbreak for the Classic is a good example of how DMCA 1201 distorts the market for technology. In the 1980s, companies saw that the public had an appetite for format-shifting their vinyl albums to cassettes, bringing their music with them into the new contexts created by the Walkman and the in-car cassette deck. The electronic industry responded by making all-in-one stereos with turntables and cassette decks that let you transfer your records to tape.
Now, the record industry hated this, but copyright didn't give them the right to stop it. When Congress created copyright, they did not transfer the right to stop the public from transferring their lawfully acquired copyrighted works from one medium to another (this was affirmed by the Supreme Court in 1984, when it ruled on the legality of VCRs in the Betamax case).
All businesses have a mix of commercial preferences (the things they wish their customers and competitors would do or not do) and legal rights (the things they can sue people for doing and not doing), and all businesses wish that more of their commercial preferences were legal rights. But legislatures don't usually make preferences into rights: first, because there are competing industries with competing preferences (the record industry's preference against home taping was opposed by Panasonic's preference for making home taping decks); and second, because business preferences are universally aimed at higher margins on their products — forcing voters to pay more for the stuff they use (lawmakers can't get elected without campaign contributions, but even more, they can't get elected without votes).
But DMCA 1201 allows for the easy conversion of preferences to rights, without any new laws being passed. All a company has to do is design its system so that its preferences are enforced by digital locks that also control access to copyrighted works (for example, a lock that prevents you from dumping your NES Classic's ROM). The thing you're trying to do (add your own legally purchased games to your NES Classic) is legal, but breaking the lock in order to do it is very, very illegal — and so your legal right is erased.
That's why we're seeing DRM in printer ink and car engines and tractors and insulin pumps and thermostats: it's a way for the makers of low-margin hardware to establish a monopoly on service, parts, consumables, and software, at their expense of their customers and their competitors — a monopoly that taxpayers will pay to enforce, through the courts.
Which is why hackers are posting arcane HOWTOs instead of founding companies that sell tiny, Raspberry Pi-based dongles for $10 that you plug into your NES Classic, feed your games to, and unplug, for a seamless, zipless conversion from a closed platform to an open one. Individual hackers may be anonymous or risk-tolerant or ignorant enough to post their instructions, but no one is going to give them the capital to start a business to do it.
That means that you, the owner of media locked up by DMCA 1201, don't get to buy that easy-to-use product to get more value out of your stuff: instead, you either take what the manufacturer has given you, or risk bricking your gadgets (a legal jailbreaking device might also brick your NES Classic, but in that case, you'd have someone to sue over it).
The NES Classic is a particularly interesting example, because the US Copyright Office has actually granted an exemption to DMCA 1201 for jailbreaking games (the Internet Archive petitioned for this as part of its games preservation work). But the exemption doesn't cover tools for jailbreaking, including instructions about how to do it, and it doesn't cover the games consoles on which the games play. Meaning that you have the legal right to jailbreak your old cartridges so you can play them on your NES Classic, and you have the right to load them onto your NES Classic once you've jailbroken them — but you don't have the right to jailbreak your NES Classic to accomplish this, and no one has the right to tell you how to do any of this stuff or give you a program or gadget to make it easy.
The same law that lets Apple decide who can make software for your Iphone allows Google to decide who can make software for your Nest thermostat allows GM to decide who can diagnose and fix your car allows Nintendo to decide who can improve your NES Classic — add a vision system and it would allow Kitchenaid to decide who can make bread for your toaster; add an RFID and it would allow Bosch to decide who can make dishes for your dishwasher.
Property is "that sole and despotic dominion which one man claims and exercises over the external things of the world, in total exclusion of the right of any other individual in the universe," but once you add software and DMCA 1201 to property, it becomes legally impossible to treat your property as though you own it: the manufacturer's dead hand lies upon your property forever, ready to punch you in the mouth the moment you attempt to use your property to your own benefit, rather than to the benefit of the manufacturer's shareholders.
According to the whiz kids at Reddit's NESClassicMods community, the solution won't work until you've created a save file in Super Mario Bros' first slot. (Chances are, you've already done this just by playing the game, since creating game saves is so easy with this system.) Once you've done that, connect your NES Classic Edition to a computer via a micro-USB cable, then boot the NES in "FEL" mode. This is done by holding down the system's reset button while pushing down the power button from a powered-off state. While you're booting, you should also run a "sunxi-FEL" interface on your computer. (An open-source version of compatible "USBBoot" software can be found here.)
The rest of the steps land firmly in "operate at your own risk" territory, as they require copying your NES Classic's internal data to your computer, then modifying and adding files via an application made by hackers. Doing so, by the way, includes the dubious step of supplying your own ROM files, which you may have either dumped from your own cartridges or downloaded from other Internet users. One tool linked from that Reddit community, however, comes with two open-source NES ROMs that are in the legal free-and-clear to upload to your hardware.
Hackers unlock NES Classic, upload new games via USB cable
[Sam Machkovech/Ars Technica]