Hi, Lori! There are some instances where the Disney Dining Plan gets a bit confusing, and you found one of those instances! Sometimes the "Disney-speak" gets misunderstood.
Fulton's Crab House is NOT included in the Base Disney Dining Plan, nor is it a signature restaurant. It is ONLY included if you have the Premium or Platinum Plan. (The "base" plan is the most common.) You may have seen "premium" or "platinum" and maybe thought "signature" which means it is included in the Base Dining Plan, but requires 2 dining credits.
HERE is the list of restaurants and Dining Plan detail for the Base Disney Dining Plan for 2010. I am guessing you viewed this page
HERE that says "Dining Plan Information" but if you click on that link, you will see that it is only for the higher-end plans that you can use Dining Credits.
It is so disappointing to have this happen. As guests, we work so hard to plan out our dining and it is no fun when there a problem with our plan. I hope this did not ruin your trip, or even your night for that matter! If you want more information, send me more details so I can help you figure out what happened! Thank you for writing to us here at the Moms Panel.