Hi Mortiz, I guess that particular screenshot method does not work on OSX currently - I should probably be able to fix that up, but there is a different screenshot method that most people use instead (since it allows for a higher resolution image than just the current screen display) which should work.

That one is here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=1049.5

Or also if you want to take a screenshot only of the current screen exactly at its current size, you could use one of the built in OSX screenshot methods like Shift+Cmd+4 to take a screenshot.

- Michael