This is really confusing. There is no 11.12. Perhaps you mean 11.10. That
is not the only source of confusion. Aptitude does not search other
distros, so a GUI isn't going to help it do what it isn't designed to do.

The only applications that would search outside sources that I know of are
Ailurus and Ubuntu Tweak, but neither is in the repositories.

If there is such an app then I would be interested in it as well. Perhaps
you could tell us what you are trying to install and we could help that way.