Hmmm two things I can think of at the moment.
Make sure you arnt putting the interface files in a sub folder of your custom folder. example:
Good: C:\..\Everquest II\UI\bobsui\ {all the xml files here}
Bad: C:\..\Everquest II\UI\bobsui\bobsui\ {all the xml files here}
Also you cant have a space in your custom interface folder name. example:
Good: bobsui
Bad: bobs ui
Thats all I can think of at the moment. You may also want to try the UI installer program by Ger.
http://www.eq2interface.com/download...fo.php?id=3529