Thanks for the question, and welcome to Apple Support Communities.
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.
I pre-ordered the Bastille album, recieved the email that it is ready, followed link, shows purchased and has been taken from my account, but not in my albums or music. I checked and I have no downloads ready. Any suggestions. If I can't get it how do I get the money credited so I can start over. No more pre-orders for me!
Same happened to me when trying to pre order MMLP2. Only took 5 minutes to figure out. On your itunes store at the top of the screen where it says your username, or email for apple ID. Click it and your account information will pop up. One of the options was saying "you have 18 songs ready for download". Click that and they will start downloading. Easy fix, though auto download the second the album is released would make more sense.