1 - Download a custom UI. Do nothing with the .zip file.
2 -
Download THIS AoC UI installer and install it. Run it as administrator. Once opened, select the correct paths (1 for the game directory, and 1 for the .zip of the custom UI you downloaded), choose the options you like and then click on "install". Done.
If you are not using StrangeUI or CorruptionUI or DefaultUI enhanced, then you can now play the game.
If you are using StrangeUI or CorruptionUI or DefaultUI enhanced, then, after you have done steps 1 and 2, go to Age Of Conan/data/gui/Aoc/ and run "aoc.exe" as administrator. You'll get a popup saying "activated". Now you can play.
Do not download the game again. If you messed up, go to your game directory, and delete "customized" file. If you were trying to install StrangeUI or CorruptionUI or DefaultUI enhanced, make sure you run "aoc.exe" as admin and get a popup saying "deactivated". Then you should be able to play the game without any trouble and to install another UI.