I agree with Mermaid_Ariel that these things are best addressed in person when they happen. It gives WDW a chance to make it right at that time and also helps assure that other guests don't have the same poor experience.
But its too late for that now, so your next best option is to write to the Yachtsman GM. The following address will get it there:
General Manager, Yachtsman Steak House
Walt Disney World
PO Box 10,000
Lake Buena Vista, FL 32830