For 7800 on MESS:
- put a7800.zip into BIOS OR a7800.ROM into BIOSa7800 (either should work; on the Mac, the "BIOS" folder should be called "ROMs")
- put all 7800 game ROMs into Softwarea7800
To start this from the command line:
MESS a7800 -cart cartname.ext
where "cartname.ext" is the name if your cartridge ROM with the extension it has (.a78, .bin, whatever)
BTW, this is all in the MESS.TXT file and the 7800 is a system which will NOT run without a cart installed, so you can't just type "MESS a7800" and expect it to pick a game out of a hat.