I've seen this question popping up here and there...or people just let their valuable miles expire. Here's what you can do to prevent that from happening in the future:
1. Go to mileageplus dining website and sign up for a free account.
http://mpdining.rewardsnetwork.com/
2. Enroll your (or account holder's) credit card or debit card to the mileage plus dining program. It can be a card of any brand (Chase, Citibank, BofA, whatever). It's free and safe.
3. Search for the closest participating restaurant near you (the closest one near me is a La Salsa fast food place)
4. Go to the closest place and charge your credit card. In my case, I could buy a diet coke for $1.
5. Your mileage plus account will be credited 3 miles for every dollar charged. Miles post in just a couple of weeks. (Or when you dine 12 times an year, you get 5 miles per dollar)
6. Your miles are good for another 18 months because of your dining activity.
I highly recommend register and enroll your current credit card anyway. Sometimes you earn miles when you don't even realize.
I went to NYC a couple of weeks ago and didn't realized that I had dined at a participating restaurant. Viola, my account was credited 250 miles today.