Nessus is built-in, hence functional by default. All you got to do is add the devices, use built-in rules which trigger incidents when matched by particualr events/sessions. You don't need to enable/disable these tools as the most recent upgrade of MARS contains most recent signatures/rules for the scanning tools.
When you click an incident, on top it will show you the RULE that was matched by the said event/session. And its not that scan is run on events, its just the rule matching process for each and every session (correleted set of events)..
