This is a window border theme for Compiz with Emerald, so I assume you have them up and running. Open Emerald Theme Manager, click Import, and select the downloaded file. Look for the theme in the list, click on it, then quit Emerald. That should be it. If you don't have Compiz but really like the theme, I can attempt to port it to Metacity (Gnome) if you'd like. I've never made a Metacity theme but I can't imagine it'd be too difficult.