I always have trouble finding my promo balances for their MP3 store or their On Demand store. Here is a kind of strange way to find it.
1) Go to the On Demand store, and go to the page for a season of a television show or something like that. For example, I believe the following is a link to Bones Season 6:
http://www.amazon.com/gp/product/B004FTSAMC/ref=atv_dp_season?ie=UTF8
which will have the link I'm about to tell you about.
2) In the middle of that page, at the bottom of the large black box describing the show, there is a bar that has links to three things in it:
Have a promotion code?
(and then a button that says "Redeem a gift card or promotion code")
View Balance
3) If you click on "View Balance", you can see any balances you have on your account with gift cards, MP3 credit, Video credit, Kindle credit, and software download credits. If you have a promotional balance, it should be listed there.
4) If it's not there, did they send you a code along with your MP3 player that needed to be applied to your account? You could apply it with the "Redeem a gift card or promotion code" button on this page.
5) If it is there but didn't get applied to your order (I recently got a $5 credit for On Demand video and I was a little nervous using it, waiting to see if it would use the credit or go to my credit card, but they did indeed use the credit. Guess what I bought? If you guessed Bones Season 6 episodes, and that's why I knew these links would be on this page, you'd be right!) then I would contact
Amazon customer service and see what they can do for you.
I believe their "contact us" link for their music download store would be:
https://www.amazon.com/gp/help/contact-us/music-downloads.html?ie=UTF8&browse_node_id=200143320
where you will be asked to log in (even if you are already logged in) and you should be able to get to someone that way. I've done online chat with Amazon support before and they were pretty helpful, but that was only one instance. Hopefully they can help with your problem too.
Good luck.