If you follow the links in my previous post, it'll take you to the wiki pages about those repositories, which include information on how to enable them on your N900. Be aware that many of the applications in these repositories (extras-devel in particular) are not suitable for end-users, and could cause serious issues with your phone. I'd advise enabling the repositories only temporarily for installing specific applications.

Once you've enabled the repositories, then any applications will show up in app manager as normal.