If both - main server and build messages processor are up and running, you should see "Nodes configuration" page in Administration area on main TeamCity server:
By default build messages processor is disabled. And all traffic produced by running builds is still handled by the main server. Once you enable build messages processor, all newly started builds will be routed to this node. Existing running builds will continue their execution on the main server.
At any point of time you can see how many builds are currently assigned to each node. If everything is configured correctly (node is accessible by agents) and there are no problems with processing builds on the messages node, eventually all of the running builds should be switched to processor node once you enable it.
Restarting servers while builds are running
Build messages processor as well as main TeamCity server can be stopped or restarted while builds are running there. If agents can't connect to build messages processor for some time, they will re-route their data to main server. If main server is also unavailable, agents will keep their data and re-send it once servers re-appear.
At the moment build messages processor node does not support plugins.