He is the method of installing .deb files using the Terminal using standard linux commands:

Installation using the Terminal

• Go into Settings -> Security -> Developer Mode and then Enable "Developer Mode"
• Go into Settings -> Programs -> installations)
• Launch the Terminal Application
• Enter ssh [email protected] (standard linux ssh command)
• When prompted for a Password, Enter rootme (Default Nokia N9 Password)
• Enter dpkg-i filename.deb

 
For information on .deb files, please see the Wiki Page