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Our article on pre-orders suggests using the Store > Check for Available Downloads menu option to download any pre-orders that are ready. Be sure you're logging in with the same Apple ID used for your purchase.
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.