[Tedric]

Edit your UI_CharacterName_Server.ini file like this...

[Main]
UISkin=Default <--- Change this to your UI folder name.

So if your custom UI was in a folder called custom it would read

[Main]
UISkin=Custom

Hope this helps!

Edit: You have to do this for each toon that you want to use the UI. Or you could edit your eqclient.txt file at the UISkin line for all toons to use it.