Download the .tgz file then install by going to:

System>Administration>Login Window

then click on the Local tab and you can just drag and drop the file into the list or use the Add button to put it in there.
Then just make sure the radio button is selected for the theme you want to use.

Widescreen version available here:
http://www.gnome-look.org/content/show.php/What%27s+Left+of+the+Soul+16x10?content=65712