ok...you got a couple of points wrong with how to install a custom UI.
Go to your "EverQuest2\UI" directory.
There will be a "Default" derectory.
If you just want to test out this one mod, then create a new directory in the UI folder. Call it "Custom" or something easy to remember.
Now copy your mod to this directory.
Now go back to the UI directory and go in to the default directory and find a file called eq2ui_custom.xml
Copy and paste this file in to your new "Custom" directory with your mod in it.
Now....open the eq2ui_custom.xml from your "Custom" directory and add in the include line to tell EQ2 about this extra mod
Code:
<include>eq2ui_custom_chatprompts.xml</include>
Now go to your main EQ2 directory and find the EQ2.ini file and open it up and change the skinname line so it reads
Code:
cl_ui_skinname Custom
The next time you run EQ2, this mod will be included.
Most are run by the /show_window Custom.Chatprompts command
I haven't had a look at it yet, so I don't know what it is supposed to do or how you want to run it (not that I am any good at custom windows
)