The installation of software from update sites (Help 
 Install New Software...) sometimes fails with an error saying something like: 
No repository found containing: 
        osgi.bundle,org.eclipse.emf.compare,1.0.1.v200909161031
No repository found containing: 
        osgi.bundle,org.eclipse.emf.compare.diff,1.0.1.v200909161031
...
This is an eclipse/p2 bug that is referenced here.
To fix this problem:
Go to Window 
 Preferences 
 Install/Update 
 Available Software Sites 
Remove all of the sites and then add them back again. This can be achieved in the Available Software Sites preference page by:
Selecting all of the update sites (highlighting all those that are checked)
Exporting them
Removing them
Restarting Rodin
Going back to the preference page and importing the update sites back (from the previously exported file)