I always run Sabayon off a live CD cause I don't have another harddrive to install it on so I will just try to walk you through the process as I would do it.
First head here: http://www.urbanterror.net/page.php?6
First thing you need to do is download and install IO Urban Terror 4.0 which you can get here: http://www.kickassctf.com/download.php?view.171
After you install that download the mod files at the following link: http://files.filefront.com/Urban+Terror ... einfo.html
Go to the directory that IO urban terror was installed at and take the mod file folder q3ut4 (I think) and place the WHOLE folder into the IO root folder so you have the following: IOUrbanTerror/q3ut4
I am not totally sure on the folder names cause I don't have urban terror installed on this PC but I hope this will help you