DarwiinRemote.app allows you to wirelessly connect a Wii Remote via Bluetooth and map each button to a key on the keyboard. Wii Remotes can be bought separately and are relatively cheap.
For a SNES emulator you’ll need to map the X and Y buttons. If you have a Wii Classic Controller, you can use it with DarwiinRemote.app as well. You may, however, need to apply this patch.
Here’s a screenshot of the DarwiinRemote setup I use for playing Donkey Kong on SNES using my Wii controller:
Note that the up, left, down and right buttons on the Wii Remote map to different directions, because I’m holding the Wii Remote as a game pad rather than pointing it to the screen (as you would do for most Wii games).
If you’re feeling retro and you still have an old SNES controller laying around, you can buy a USB Super RetroPort which essentially adds a SNES controller port to your MacBook.
If you don’t have a SNES controller, you can buy a USB Super RetroPad which is a SNES controller that plugs directly into an USB port.
P.S. I’ve posted the same question on gaming.stackexchange.com two years ago (back when those questions were still allowed).