Load your label design into Acoustica CD/DVD Label Maker and click on the Print button. Look at the small "print preview" area and, if you've chosen the correct Paper Type, you should see representations of the bonus spines and cores.
Clicking once on a spine or core will select it for printing. It might be hard to see (depending on your screen resolution and your label design), but the outline of the spine or core will fill in with a small version of whatever you put on the spine or core. Clicking it again will turn it off. You can choose to print more than one at once just by clicking on them.