I have found a bug inside the SMF 2.x Admin CP
You can chose either classic or the cool drop down menus, and there is always a link to switch the theme type. However, these are linked incorrectly. The URL that links to the other menu does http://yourforum/index.php?action=admin;area=CURRENT;sc=SESSION (http://yourforum/index.php?action=admin;area=CURRENT;sc=SESSION)# but it should not be sc=SESSION it should be sesc=SESSION :)
I hope I wasn't confusing >.>
Thanks, I've logged this now:
Bug #2037: sc=SESSION not sesc=SESSION inside admin panel? (http://dev.simplemachines.org/mantis/view.php?id=2037)
This has been fixed in the SVN.