If you press the Song on iPod button, CantoPod access your iPod and list you all the songs in your device.
Once you see all of them, you only have to choose the ones you want the lyrics for and it will do the rest.
If you want to perform it manually, you will only have to type the title of the song and the artist and it will be shown on screen.
Remember, if you are a music lover and own an iPod, CantoPod is a must have app.
You can find a Mac version in the Official Site.