Looking through these forums, I see alot of posts with people complaining that the UIs they download off such websites as Curse.com | Downloads Forums News Blogs Guides Videos Screenshots | Curse do not work, and in no way resemble the lovely looking UI picture that the author has included.
This is because you are not properly installing the UI - it requires a little bit more than simply dumping the files contained in the RAR into your WoW folder.
When you open a RAR file containing a UI, it will look like this;
After you have copied these files into your WoW folder, it's time to fiddle around with the WTF folder.
Upon opening the WTF folder, click on the icon labelled "Account". Then, rename the file here (often titled "YOURACCOUNTNAME") to your own account name, in BLOCK CAPITALS.
Then, open the account name file, and rename the file titled "YourServerName" to your server name; this time capitalise the first letter, but leave the rest in lower case. Open this, and rename the file here to your character name.
You can now boot up WoW, and everything should be in order.
If it is not, however, you need not panic. You need to find out which addon is causing the problem, and make sure the profile is set to the name of the character who created the UI, and not to "default" or your character.
A useful addon for checking profiles isDeuceCommander - WowAce Wiki , which provides a graphical interface for the commands of all Ace addons.
Well. That's a guide. Enjoy, try to hold off flaming too much, etc.