Go to the Left Sidebar, by either mousing the left edge of the MusicBee window or dropping the Configure Layout menu (button to the left of the "Search" field in upper right corner) and choosing Left Sidebar-Show Panel.
Then clicking on the choices you see there will open them as a tab (History, Music, Now Playing, Music Explorer, etc.). If you don't see the choice you wanted there, go to Preferences/Layout(1), and check the boxes in the "Left Sidebar: Navigator" part on the bottom.
To keep from accidently closing the tabs again, right-click the tabs you want to keep there, and choose "Pin Tab."
We're nothing if not thorough here, haha.. Enjoy using the Bee!