Click Media Library, and then expand My Playlists.
Click the playlist that you want to modify.
Click the Playlists button, and then click EditPlaylist.
-or-
On the File menu, click Edit Current Playlist.
In the View Media Library by list, click the category you want to add items from.
In the list, find the item you want to add to the playlist.
Clicking an individual item adds it to the playlist.
Notes
In Media Library, when a playlist is selected, a playlist menu icon is displayed at the bottom of the screen in the Media Information pane. You can click the icon to display a playlist menu which contains the following commands, EditPlaylist, DeleteSelected, Clear, Sort, Open, SaveAs, and CopytoCDorDevice. The playlist menu icon is also available in Now Playing.