I just hold the apple key (command), shift, and 3. Just like you said. I'm not sure why it wouldn't work for you. Try searching mac help from the desktop for 'screen shot'. If you're using system 10 to play Nova, the commands could be different in that OS. I'm using 9.1.
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(url="http://"http://www.polytone.net/new_axiom")nova: the new axiom(/url)
(url="http://"http://www.polytone.net")polytone.net: exploring a new era in music(/url)