[Werepuppy]

I don't know if it will show this sort of credit, but if you go to the digital music section, and then look at a page for a song or album, any will do, underneath the buy button look for the link that says "Add gift card or promotion code", you may need to click on the "More options" link first.

This will bring up a box with your gift card and music balances. I know that when I had some promotional credit for the app store it was displayed in the music part of the credit so you may find it displays your credit there too.