[Agathorn]

If you unzip to a folder named "Serenity" inside your EQ2/UI folder, say somethign like this:
"c:\program files\sony\everquest ii\ui\serenity"

Then your EQ2.INI file should look like this:
cl_ui_skinname Serenity
cl_ui_subdir UI/

And the EQ2.INI file should be located in:
"c:\program files\sony\everquest ii"

A common mistake is to put that file in Serenity or in UI. But it shoudl be in the base EQ2 directory.