The syntax in the query designer is something along the line of;
MyCalcField: IIf([FieldA] = True, 0 , 99)
Where 0 is the true result and 99 is the false result.
You could replace 99 with another field if it contained the false value you wanted;
MyCalcField: IIf([FieldA] = True, 0 , [FieldB])