Don't be so sure that the USB flash drive is OK -- at least, not if you define "OK" as "OK for a Mac." Apple uses EFI 1.1, whereas PCs use EFI 2.x (aka UEFI). What's more, Apple deviates from the standard EFI boot mechanism, so efforts to create EFI-bootable media that work on both PCs and Macs often fail. To make matters even worse, there are significant model-to-model differences between Macs and PCs. It's even worse in your case, since you've got a 32-bit EFI. If you're trying to boot a 64-bit distribution, it won't boot in EFI mode, even if you've got a 64-bit CPU, since a 32-bit EFI can't handle the 64-bit EFI binaries that are included on 64-bit distributions. If you've got a 32-bit distribution, chances are it doesn't include any EFI support. Thus, the only way you'll get that USB drive to boot on your Mac is to boot it in BIOS mode. In theory, rEFIt can do this; but as I've just outlined, there are enough weird problems that you may be forced to do it some other way.
I don't know about Mint, or even the very latest version of Ubuntu, but in the recent past, Ubuntu has released a special Mac version of its distribution to work around these problems. Ironically, this version omitted the EFI support, since it's easier to get the installer booted in BIOS mode. You might look for such a version of Mint. If that fails, look for a directory on your USB flash drive called EFI/BOOT (it may vary in case from this) and delete or rename it. That will remove the EFI boot loaders from the equation, leaving only the BIOS boot loader.