Mubase:
What else could it be????
Oh I don't know ... maybe its just cos its a Mac? 
If it's working on windows and the emu that would say to me the installation of arduino software got currupt or perhaps your version of OSX is not supported. I'm not sure what the arduino uploader is designed in as I haven't even used it yet , but if it is supposed to run on all oses , it may be a java app? Then again I don't know if Java even provides that kind of functionality and I doubt it. In either case , maybe a java update will help?