I'd recommend you to convert those files to lossless .avi ones, because there's no special codec required for playing those usually (I reckon it's the same on a Mac, but I'm not giving you a warranty at all lol). For the converting I'd suggest you to use VirtualDub or some similar compatible software in case you want to do that on a Mac OS (e.g. - Avidemux, though I heard it's slightly bugged).