You can also go to the OpsManager details page for that controller, scroll to the end and click the "Refresh" button in the "Monitoring" section.
From the command line, you can execute "dfm host discover controller-name". That does the same thing.
Out of curiosity, where would you expect to find a "Refresh" button in NMC? We've been talking of adding one and I want to ensure it shows up in the right place.
-- Pete