In iPhoto, sometimes after you select the option to add music to a slideshow, you see your playlists but not your individual songs in the dialog sheet.
If this happens to you, try resizing the sheet by dragging the lower-right corner to make it bigger. You should eventually see a divider that separates the playlist pane from the song listings pane. Once you see this divider, click and drag it upward so that you can see items in both parts of the window more easily.