This is most likely an incompatibility between the EFI shell you're trying to run and your computer's firmware. I recommend you try other EFI shells. One that comes with rEFIt is most likely to work on Macs (especially older ones), but there are a number of additional links here and elsewhere. (Try doing a Web search for "EFI shell binary" or something similar.)
Unfortunately, every EFI shell binary I've run across has problems on some computers, so providing one that's universally usable seems to be difficult or impossible. Note that Macs have 1.x versions of EFI, which means they're most likely to work with the older EFI shell v1 rather than the newer v2