Open on your N900 web browser the following page :
http://wiki.maemo.org/Community_SSU/#Installation

Click on one of the top green arrows ("stable" or "testing") two times with the same choice :
- the first time will install the Community SSU application catalog ("stable" or "testing" version) in the N900 application manager (close the application manager once it is done)
- the second time will start the Community SSU installation "initialisation" application from the N900 application manager (only a few KB)

Once it is done, you should have a new icon appearing on your application list. Launch this new application : it should open a terminal and install the Community SSU (this time, the download size is about 27 MB).