Well if it doesn't open up with the Application Manger, then you need to open Xterm, become root, and dpkg -i /home/user/MyDocs/.documents/pidgin*.deb (assuming you saved it to the default Documents directory).
It would be easier if you just added extras-testing to your repository list (blah blah this is dangerous as software in repo has not been tested).

Here's how:
http://p7230.homelinux.com/os2008/da...00testing.html