Emerald is the theme manager for Beryl and Compiz. You need to activate one of those to use Emerald. You can run Beryl from the Cassandra menu, or open a terminal and type beryl-manager.
After that a red gem should appear in your system notification area and right-clicking on that will bring up the menu. From there you can just go and play. There is some VERY cool stuff to be discovered.
Note that Beryl and Compiz do take a fair amount of computing and graphics power to do their thing, so if you are running older/slower hardware it could impact your system's performance.