Yeah, I remember now why I didn't install it before.
To get it working on a Mac is utterly ridiculous. You have to install DarwinPorts/MacPorts in order to install the emulator. But in order to install DarwinPorts, you have to install the Darwin Developer Tools from your OS installation discs. Developer Tools is 3 gigabytes.
That's.... f*cking ludicrous. There's no way in hell I need to waste 3 gigabytes of disk space to install a damn emulator. IMO, the author of mednafen (or the party responsible for the MacOS port) should offer a precompiled binary for download. The number of people wanting to install mednafen that already have Darwin Developer Tools installed is probably ~0. I've never, ever encountered an application that required the user to jump through so many hoops just to complete an install.