Fallout 1 for Mac is almost certainly Fallout 1 for PowerPC... and that's not what Macs use anymore.
Does what you bought expressly say it works on Intel x86? (I don't expect it to, but I can't know if they re-released it with Dosbox.)
To get that to run, you would likely need a PPC emulator ~that's a hassle. The better option (but not the one you want to hear) is to find the original CD for that that includes both the DOS and Windows versions. The DOS version, you can get to run on a Mac via DosBox for free; while the Windows version will run on either an emulated windows install, or via dual boot. *Macs use PC hardware now, and no longer support the PowerPC based software.
*Alternatively (but hardly practical), you could try finding a PPC Imac cheap. I've found them on the street before, and that would run it if it ran OS9 or before. MAC OSX dropped support for OS9 emulation with 10.4
I think.
**An interesting bit of trivia about Fallout 1 for Mac: The save games are completely compatible between the Mac & PC versions despite little-endian /big-endian architecture. You could save your game on PC, and load it on a Mac, and vise versa.