Heya all,
I had the same problem upon using the store for the first time when I installed iTunes a few days ago. Support ended up sorting out the issue. I've cut & pasted their instructions below.
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I am sorry that you still have not been able to finish setting up your account on the iTunes Store.
In order to use your account on the iTunes Store, you will need to change a few settings on your Apple ID via the MyInfo website.
1) Log into your Apple ID at
http://myinfo.apple.com
2) After logging in, choose the link "Apple ID" from the left-hand sidebar.
3) Once on this screen, you will need to choose a security question and also set your birth day and month.
Once you have followed these steps, you should be able to log into the iTunes Store and successfully set a method of payment.