I know, and I think it's all there for you. But you need to implement the userinterface.

If you already have your buttons and everything used to scroll thru the list of available songs list you should be set. When you press "select" you set the Song-variable to whatever number the user has selected from the list and then GOSUB AddSong, the song is added to the list.
Code:
If UpButton = 1 Then Song = Song + 1       ' Point to next available song to chose from.
If DownButton = 1 Then Song = Song - 1    'Point to previous available song to chose from.
 
If SelectButton = 1 Then GOSUB AddSong    'Add selected song to list.
 
If RemoveButton = 1 then
  AddPointer = AddPointer - 1  ' Take one step back in the list
  PlayList[AddPointer] = 0       ' Remove last entry from list.
EndIf
If this is not what you mean I'm afraid I don't understand exactly what you're trying to do.

/Henrik.