Same problem here.... I don't have a credit card and never will, So, I purchased a $15 iTunes card. I installed 10.6.6, launched the App Store, signed in with my Apple ID, read the T&C which did not specify that a credit card was needed to redeem the cards which it does say are accepted forms of payment. Agreed to the T&C, continued to the main page of App Store and clicked link for iMovie '11.
Price listed as $14.99. Clicked 'Buy', logged in, page loaded with input for credit card, gift card code and previously registered account info. Entered the gift card, clicked continue....
Clicked again when prompted to redeem gift card and was returned to main page of App Store - No download of application occurred.
Clicked link for iMovie '11. Clicked 'Buy' presented with "Insufficient remaining store credit" and only option is to log in(again) and enter credit card info.
In App Store Help under "If you're having trouble", followed these instructions: If you’re still having trouble, click Support in the Quick Links section. Upon clicking 'Support' link in quick links of App Store, this page comes up:
"Hmm, the page you’re looking for can’t be found." NICE!
SUMMARY: Redeemed $15 iTunes card. Account showing $15 balance. Click to purchase $14.99 application. Unable to purchase and download app. Support link bad. No readily available contact information for App Store support to be found.
Needless to say, I am EXTREMELY disappointed with the "Mac App Store".
JP