If you click File->Library->Open Library, MusicBee will show the folder path of its library (the file "MusicBeeLibrary.mbl") location. However, it sounds like you're actually trying to find from what locations on your drives(s) you have added files to your library. You have to tell MusicBee from where you want files added to MusicBeeLibrary.mbl, either by adding them manually (File->Add Files to Library) or via an automated procedure depending on what you have entered in Prefs->Library->Monitored Folders.
Under the Music tab in the main panel that lists all the files in the library, right-clicking the panel header, choosing "Set Displayed Fields", and then adding "Path" from the left column to Selected Fields on the right will show the full file path of the actual music file.