Well...
Since it works on one computer, then it obviously works fine on the server. So that narrows down to being outside the realm of PHP and this forum.
I dont know alot about Macs, since they suck (in my opinion), but the best thing to do, in my opinion, would to be to get the latest patches and browser updates for the machine that doesnt work. If it still doesnt work after all of the updates have been made, compare what's different between the machines. Browsers and plug-ins only really.
If all else fails, get a PC, then I can help better.
🙂