I have the same issue. Everything was working find on lion, but after I installed Mountain lion it stopped working.
I deleted the application and reinstalled it, but that did not make it work. I get the login screen, but when I press play the screen goes black.
Installed GW2 on Macbook Pro running Mountain Lion. Was able to log in, select server screen came up and then black screen and only mouse cursor for game showing. Alt tabbed out to desktop, quit GW2. Restarted mac and GW2, now only see black screen and mouse cursor with music playing.
Don’t have a windows pc at moment, waiting for new computer replacement with apple. Any help to use mac client would be much appreciated
System specs are in attached image