I had trouble activating the plugin after downloading it. I was getting an error saying that there was a syntax error in 'gecka-submenu/models/NavMenuHacks.php' on line 130.
It turned out that the problem was just that the file 'NavMenuHacks.php' started with a short php tag (ie <? instead of <?php). My php installation was not configured to allow the use of short tags. Changing the file to use the full php tag fixed the syntax error. It is advisable to change this file to use the full tags, rather than changing your PHP configuration to allow short tags, as short tags can interfere with XML documents and shouldn't be used anymore.
Hope this helps someone.