Build messages processor uses the same approach to logging facilities as the main server. You can check how startup is going in teamcity-server.log file.
You can also open <processor URL> in your browser, there you should see regular TeamCity startup screens.
Enabling build messages processor
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.
And vice versa, if you decide to disable messages processor, only newly started builds will be switched to the main server, builds that were already running on the messages processor will continue running there.
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.
At the moment build messages processor node does not support plugins.