Pre-orders generally seem to become available between 12:01 and about 12:15 AM Eastern on release day.
In your B&N Library, make sure that either your WiFi or 3G connection is active (i.e., that you're not in Airplane Mode) and hit "Check for New B&N Content" and you should see the book there. Highlight the book and go to "Show Item Details and Options". If the first item is "Read", then the book has already downloaded; if it's "Download", hit that and it should download immediately.
If after checking for new content you don't see the book, call Customer Service at 1-800-THE-BOOK (1-800-843-2665), Option 2, Digital Support.
Enjoy your Nook.
I had the same problem. When I would click check for new content, the pre-order would still not show up.
I had to go online into My Library and archive the book then redownload it. When I went back to my Nook and checked for new content, the new book showed up in My Library.