Adding plugins for iTunes play lists

How do I add my playlist that’s on my iPad to Plexamp

Short answer: you can’t.

Long answer: Plexamp is not a media player.
Plexamp is a “Plex client”.

It can only play audio files which have been added to a Plex Media Server.
In order for to play a playlist, this playlist and the included audio files must have been added to the Plex server.