They should be completely compatible. And the extension doesn't mean anything, it's the internal format of the file that makes a difference. The extension is just a cheap way Microsoft came up with for creating program associations, rather than do a proper job of it and use metadata embedded into the file.
Office 2008 does lack support for VBA, which may be causing problems. In which case, you'd want to try rolling back to Office 2004 which still has VBA.
That should really be the only difference between the two aside from how they look. They should otherwise have feature parity. And technically Office 2008 has AppleScript support which provides much the same purpose as VBA, it's just only supported on the Mac side.
Finally, Mac is not an acronym, it's short for Macintosh... Like the type of apple... Get it? So it doesn't need to be in all caps.
1. What are the file extensions for MAC excel? Do they differ from PC excel?
2. What are the differences between MAC excel 08 vs PC excel 07?
I have had some work done by a provider in elance in PC excel 07 and now i can not read the spread sheet in my MAC environment. The provider is saying that MAX and PC excel versions are 100% compatible - I disagree and need to know the differences but I am not tech/program savvy.