Romhustler generally only provides roms who’s publishers aren’t making money on the corresponding games. That wouldn’t be a problem except that Ninty is now selling N64 games via download on the Wii.
Yeah the roms on there are for the emulator ‘Project64’ which is a windows app, sorry… Not sure about N64 emulators for mac, I haven’t found a good one yet :(
As far as I know most rom files aren’t associated with any particular operating system. It may be the case that your emulator isn’t associated with the file format that might find on romhustler. try opening the rom with the emulator rather than in finder.