Back to Emulation
Surprisingly, emulation on macs is fairly easy. There are lots of developers working on projects for mac, and emulation is a well supported task.
The best site for mac emulation software is http://www.emulation.net You will want to scroll all the way to the bottom of it where they have something that looks like finder and click on the emulation icon. It will fold out to reveal the different systems. (It took me a little while to figure this out :)
Most popular emulatorsEdit
Now, if you are doing emulation or any gaming on a mac you should pick up a usb controller. It makes gaming a lot more fun then using the keyboard and mouse. A nice cheap one that I use is the Belkin Nostromo N45 Dual Analog USB Gamepad.
Controller Set UpEdit
Another thing that is REQUIRED if you are using a controller with a mac is The Gamepad Companion. It is a piece of software that will map keystrokes and mice to controllers, so if you set J to be the jumpkey for a game, you can set a button on the controller to be mapped to J, so pressing it will make your character jump. You should configure your gamepad to match the controls of MacMame and then configure your other emulators to match that. The reason is that there are so many different configureations of controllers possible for mame that its best to go with the default and then bring the other emulators into the same line.
One more note on controllers, you can have any number of them plugged in at once and have each one mapped to a different set of keys, so you can play multiplayer games.