In the Tarrasch menu, try clicking on one of the database menu choices, like "position search."
If that doesn't work, tell Tarrasch where to find the Millionbase by clicking on "select current database" in the drop down menu of the "database" menu. (Database->Select current database).
Or try clicking on the envelope icon in between the green recycling arrows, and the bug, on the menu bar.
It should open in a separate window.
The Tarrasch GUI is now in beta version 3 and has a 2.5 million game database. It looks interesting so I downloaded the GUI and database but can't seem to figure out how to open up the games in the database. Is a special program required?