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Depending on your settings, the songs may already have been downloaded to your iTunes library.
The article below will explain how the Pre-orders process works.
If you still cannot find this music on your computer, the iTunes Store Support team will be happy to help you with your order.
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 Downloadsto begin downloading your pre-order.
I hope this information helps ...
Have a great day!