In order to download your pre-order, you'll need to click the download link in the e-mail you've received indicating that your pre-order is available. Alternatively, you can choose 'Check for available downloads' from the Store menu in iTunes.
Download a pre-ordered item
When a pre-ordered item becomes available, you will receive an email letting you know that you can download your pre-order.
If you enabled the option to automatically download prepurchased content in iTunes, then your pre-ordered content may already be in your library. If not, click the download link provided in your email notification, and your pre-ordered content will begin to download. You can also use the Check for Available Downloads feature in iTunes on your computer (choose Store > Check for Available Downloads) to begin downloading your pre-order.