[Reven]

there is one trick you could try and that is making the UI you want to use the master UI thats done by loading the UI into the C:\Documents and Settings\Mine\My Documents\The Lord of the Rings Online\ui folder instead of the sub-folder "skins" if the UI still doesn't appear on your UI list after you start the game back up try the following more drastic measure.
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nuke your UI folder and starting it again so your path would be
C:\Documents and Settings\Mine\My Documents\The Lord of the Rings Online\ui\skins\

then unzip the packaged UI of your choice into skins so your path looks like
C:\Documents and Settings\Mine\My Documents\The Lord of the Rings Online\ui\skins\uifoldername

now back into the C:\Documents and Settings\Mine\My Documents\The Lord of the Rings Online folder and find your settings INI and just delete it, let the game make a new one with the UI folder already loaded.