Thanks for the question. I understand you are having issues downloading a pre-ordered item. 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 have the same problem, with the I album I purchased months ago, that should have been available for download as of yesterday. I got the e-mail "Your pre-oder for "Almah" is now available", but if I click the link I'm asked to sign in twice, then nothing happens.
If I browse to the album, it says "Purchased", but there is no download button. Also, the songs are not in my library.
I'm confused, I paid for the music, so where is it..???