In LW go tools > options > player & make sure that 'use LW player' is ticked > Apply > ok. If the player still doesn't show up, restart LW.

If it's still not showing, try this: Delete LW's preferences folder when LW's properly closed. See point 3 here & also point 2 if you've used older versions of LW in the past (there could be a folder in either location).
Fixes for LimeWire
This will kill all of your settings within options, you'll have to go back through afterwards & reset things. If you're not sure of your settings, go through LW's tools > options & write them all down, so that you can reset things later.

The LW player is pretty horrible, you might find it easier to make something else your default player