Thanks for the question, I understand how frustrating this may be. The following article outlines the procedure of pre-orders and redeeming the download:
Downloading a pre-ordered item
When a pre-order item becomes available, you will receive an email letting you know your pre-order is available for download.
If you've enabled iTunes to automatically download prepurchased content, then your pre-order content may already be downloaded to your library. If not, simply click the download link provided within your email notification, and your pre-order content will begin to download. Alternatively you can use the "Check for Available Downloads" feature in iTunes on your computer by choosing Store > Check for Available Downloads to begin downloading your pre-order.
However, if you are still having issues after reviewing the above information, please see this resource for infomration on reporting an issue with your purchase: