Theater Mode plugin is by default enabled when MB is installed. But sometimes that's not the case for various reasons. It happened to me as well when I installed MB first time. Then you will need to enable it manually.
1. Go to Edit> Preferences> Plugins> Theater Mode> Enable
2. To activate theater mode, restart MB, go to Main menu> View> Theater mode, and select a theater view in the list
3. Sometimes you may get an error message that the plugin might be blocked. In that case consult the following thread:http://getmusicbee.com/forum/index.php?topic=6608.0