Actually if you have an application you purchased and the following applys:
1). Application is corrupt.
2) Application does not perform as per description on iTunes
3) Application does not suit your geographic area ie Australia eg USA maps and no mention of this was in iTunes description
4) You purchased the application in error by selecting buy and no request came up for password as you had bought an app just moments before.
Include your user name, your full details, the application name, the order number with date and the reason for requesting a credit
You will receive a response with in 48 hours then receive credit with in 5 days if it is a credit card purchase if you used an iTunes card I believe they issue credit to you account.
To locate your order number on your computer log in to your iTunes account, Account, recent purchases then look for the Application that you require a credit for. The order number and date is next to the Application name.
This links below will assist in contacting apple read this one first.
http://support.apple.com/kb/TS1702
http://support.apple.com/kb/HT1725?viewlocale=en_US
iTunesStoreSupport@apple.com. This is the link to report your problem