Mupen64 is a Nintendo 64 emulator ported for the Mac. It was ported by Adam Green and Lamer0. The Mupen64 project was discontinued in 2005, and some developers continued this project under the name of Mupen64 plus. I will be writting about Mupen64, though.
Mupen64 doesn't really have a graphical user interface. When you first open the application, it will ask you to open a ROM file. Once you do that, the game will start. You can access the application's settings from the menus on the Menubar. There aren't really a lot of things that you can change. You can modify the resolution, the texture cache size, and you can enable fog, fullscreen mode, 2xSal Filtering and Bilinear Filtering.
I don't see any way to customize the keyboard controls or to use a USB gamepad.
In my testing, I opened two Nintendo 64 ROMs that I knew worked fine, because I tested them with other emulators. Both ROMs were opened but the application crashed very soon. All I really got to see was the Nintendo logo. I am running OS X Lion, and this app was last updated in 2005, so that might be the reason. It probably would work on older computers. If you really want to test the Mupen64 emulator, try the Plus app, to see if that one works.