It might be because you have XenPorta installed...
There's a bit of a bug in XenForo which has been resolved in XenForo 1.2.
Basically any add-on that changes routes overwrite each other.
So the solution in the interim is to use Jake's Route Changer add-on.
With that add-on you would need to disregard any options in the Gallery or XenPorta and set up all the route changes in the Route Changer. This might take a few posts to get resolved so if you post over at the Customer Area of
http://xenmediagallery.com that will be useful and I'll post the steps to produce the solution here later
EDIT: With regards to changing the navigation tab, look for the phrase "xengallery_media" and change it to whatever you like.