McBees: Sorry about this, how frustrating to learn about this after the fact. You should register for your own SkyMiles account as others have said. Also, I believe that you can request credit for a past trip for up to one year, though I'm not positive about this policy. It is worth looking into. To do this:

1.Create your own SkyMiles account (if you have not already)
2. Login to www.delta.com
3. Click on "my skymiles" (under the My Delta banner on the homepage)
4. Click on "my account activity"
5. Click on "Don't See Your Trip Listed? Request mileage credit now" and follow the instructions.