It seems like this should be simple, but I can't seem to find the right place to click.
I see how I can go into edit mode for a playlist and remove songs. I just can't figure out how to add songs to a playlist on the device. Ideally I'd like to be able to do it from the now-playing screen somehow, but any solution is acceptable as long as it doesn't involve connecting to my computer and using iTunes to do the add.
On the device, in the tab bar, select Playlists, then select the playlist you want to edit. Press the Edit button, then in the top right corner is a plus sign. Tap the plus sign to add tracks to a playlist on your iOS4 device.