Go to Getdeb.net. Download the deb file. Click on it. Give your password and it
is installed.

http://www.getdeb.net/search.php?keywords=second+life

BTW, this is a great source for many programs not found in the repositories such
as Ubuntu Tweak, Songbird, Wine-doors, and it often has the latest versions of
many programs such as Wine and Gimp.