I have never had a problem using brushes made on a Windows machine in Photoshop on a Mac. For that matter, I've been able to use brushes made for Paint Shop Pro in my Photoshop. If you're still using v9.x, change the Type/Creator to match that of a brush that came with Photoshop. For 10.x users, make sure the extension is (I believe) .abr. Check that, because I don't have that Mac at work and it could be different.