Please note that this will only work if

1) the file(s) associated with the album or single track actually 
exist and are accessible by iTunes (in iTunes list display mode, check 
for a small exclaimation mark in front of a track you can't seem to add)

2) You are playing music from a playlist. Otherwise, the active 
playlist can't be determined and it's not clear to which playlist the 
tracks should be added.

Best regards, 
Felix