Update: you can no longer to this since they ripped out the old code.
YouTube gimped your access to the old playlist — here is a hax to get to it.
here is the URL to the OLD playlist editor removed. youtube.com/my_playlists?p=”PLid#”
1. goto “channel” click on “Arrange Playlists” then “save” (this takes you to the OLD playlist id #.) I haven’t checked to see if the new playlists have the same or different PLid#.
2. u’ll see your playlist id in the URL. Copy it.
For example if this is my playlist youtube.com/user/FOUNDUPSTV#grid/user/6E531AEA8F0D5486 THEN “6E531AEA8F0D5486” is my “PLid#”. JUST ad the ID without ” “.
3. Hit enter and then “edit” and you are there.