I see that you have integrated your phpbb3 in a joomla purity II template.
Could you tell me how you have done it?
Copy the source code of your menu
into your phpbb3 template and link the mega.js and mootools.js into your phpbb overall_header.html template.
Next you will have to add the styling of the menu into colors.css theme file.
I would like to assist even further but my time is pretty limited. See the source of my website's forum to make sure your work is done right.