highlighting multiple songs for moving into a playlist

Hold down the Apple key when you click to select multiple songs not in a row together. Hold down Shift key and click to the select a group in a row. Grab the album icon and drag to the playlist to add an entire album.

If the songs are all together, all the songs in an album for instance click on the first one to highlight it the hold down the shift key and click on the last one then drag the selection to your playlist. If the songs are not in order but are scattered through your library hold down the Command (⌘) key and click on each of the songs you want to select then drag them to the playlist.