Cannot install shell-themes using gnome-tweak-tool

How do I fix this? I will attempt to upload a screenshot of the problem. Notice there is an error like triangle near the shell themes and nothing is select-able. Also there are no shell extensions under the shell extensions tab. I have come across many postings on how to fix this but none worked for me. I currently have 12.04 LTS. I have a custom Phenom quad core machine with Radeon HD 5770 graphics if that helps.

If you are using gnome-classic session, You can't enable shell theme, simply because you aren't using gnome-shell.

Login to Gnome session, which uses Gnome-Shell. Install the user-theme extension by going to this page. Close the gnome-tweak-tool and re-open it. You should now change the gnome-shell theme

You should also install gnome-shell themes to be able to select a custom theme than the default one.

See this question: How do I get and install more themes, icons, and pointers?