To use Mesh, you need to unlock it first. If you use BYOB, then just choose "Host Mesh" or click "Join Mesh" and join with a local network user's IP address (ask them or it!). However, if you don't have Mesh, look at the instructions at http://scratch.mit.edu/forums/viewtopic.php?id=32046 .
On Mesh, you can see Marble scores from others in your Mesh session as they score them! So far that is the only thing you can do on Mesh.