If you are in the desktop version on a mac, if you press Command+A, that will select all the tracks. Then while holding Control and Command, right click, which will give you the option to add all selected tracks to what ever play list you want.
If hitting Ctrl and A doesn't work. Click on another folder, then back to songs and try Ctrl & A again and it should work. Hope that helps! PS. This can only be done on the downloaded version on the computer.