Recheck your Cryptic STO account, and look for "Star Trek Online-- Pre-Order". If this is listed, you are all set for head start. If not, you may need to contact D2D.
EDIT: Sorry...I mistook your "Digital Download" as "Direct2Drive Digital Download". Did you order this through Atari, Gamestop D2D?
I ordered the Digital Download from D2D and the pre-order key opened up not only the beta but the head-start as well. Sorry, I can't speak for Atari or Gamestop, but if something is goofy, I'd contact the company directly.
If you purchased the game through steam there is no way to simply view your key, the way to activate your key is the following:
Note: make sure you're logged in to the STO website before doing this.
1) Select Star Trek Online - Beta in your "my games" list.
2) Open the properties tab by either right clicking the game or clicking the properties button on the bottom right of the Steam window.
3) At the bottom of the "General" section of the properties tab, click the "Visit Star Trek Online - beta account web site" button. It takes you to the page with the key already typed out. Then hit accept.
It will then take you to your dashboard, if "Star Trek Online - Preorder" is listed as activated then everything is ready for the headstart.