If you're trying to use an iTunes card, just go to the store link located at the top, select view my account and select edit payment info. Where it asks for a credit card, select "none" and another option will appear asking if you have a gift card/certificate to redeem.
I uninstalled version 10. It was slow & took forever to search for songs. I seemed to get real crappy results too when I searched. Went back to version 8. It's faster, and I get way more results when I search. Anyway, since I did this I also get the "temporarily unavailabe" message when I try to access the store. I can search by artist, song, etc., & download & purchase music. Just can't use the home page, which is fine with me. I think it wants you to update to the latest version of iTunes to get to the home page. Would rather have the better search results & run much faster & smoother. I think the only thing we're missing out on is the home page always displays the latest releases. Otherwise, I think you can work around it and still access everything else. Hope this helps.