The main window is divided in two parts:
Left side: plugins list view
Right side: documentation
The "Installed" group displays the locally installed plugins. The other groups display the plugins available on servers.
The set of available servers can be changed in Menu
Servers (inside the plugins window).
One server address is for instance http://tulip.labri.fr/pluginsServer/server.php .
Click a checkbox to select a plugin for install/uninstall. Click the version number to see the documentation.