ProB 2.0 for Rodin 3.1 can be installed from the update site located at:
http://nightly.cobra.cs.uni-duesseldorf.de/prob2/updates/releases/advance-final/
The installation follows the normal Eclipse installation procedure. Choose 'Install New Software...' from Rodin's Help menu.
Enter the update site into the textfield and hit enter. The result is shown in the following screenshot: