You need to get a form DS-3053 and fill items 1, 2, 3, and 5 (Statement of Special Circumstances), if you are an applying parent or guardian and the written consent of the non-applying parent or guardian cannot be obtained. The statement must explain in detail the non-applying parent's unavailability and recent efforts made to contact the non-applying parent. Attach the court papers or notarized copies to the application.
http://travel.state.gov/passport/forms/ds3053/ds3053_846.html ..