A new mac demo has been created now. Hopefully it works on older versions of OSX as well. Please give it a try and let me know how it works. If it works ok I'll tell Shrapnel to update the download page with this one.
http://ulm.illwinter.com/dom3/dominions3demo_osx.zip
edit: oops I posted as the big head, but I'm really the giant toad.