Q: I got an email saying my pre-order is ready to download, but it doesn't show up in my purchases on my iPhone

Hello there, Christaen.

The following Knowledge Base article should provide some useful information on how to redeem your purchase:

iTunes Store: About Pre-orders

http://support.apple.com/kb/HT5714

Specifically:

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.

If the issue persists, please review:

iTunes 11 for Mac: Download previous purchases from the iTunes Store

http://support.apple.com/kb/PH12283

Thanks for reaching out to Apple Support Communities.

Cheer,

Pedro D.