I know how to create a PlayList in the API, but how do I actually add Media entries to the playlist?
Unlike Category, there is no KalturaPlayListEntry object.
The content is on the playlist object itself. Either by filter (for automatic), or the list itself (for manual)
see here: http://www.kaltura.com/api_v3/testmeDoc/index.php?object=KalturaPlaylist
If you want to understand better, do reverse engineering: get a playlist you created in KMC and view its content