That may be due to either of 2 reasons I can think of. 1. The LW player has been disabled. Go to LW's menu bar, Tools>Options>Player & make sure option is ticked. You might need to re-open LW before this has an effect on the playlist window section.
2. (a) Click the black arrow at the bottom so it shows both the Library & the mp3 playlist. Clicking the black arrow will show one or the other or both. (b) Or Drag the partition up so it's not hidden as shown below with the red arrow. (btw I changed the title of your thread.)