How to add items to and modify an old playlist?

1. To add a new item to an old playlist: Open the playlist panel, select (click) an old playlist. The playlist items should be listed in the view panel below. Open the collection panel and drag any item (from non-star area) and drop to the view.

2. To modify (re-order or remove): On the view panel, highlight an item and then click "x" to remove, "up" to move up, "down" to move down, "s" to shuffle accordingly. You could also right click an item and follow the popup menu.