I have a HD12538 weedeater lawn tractor. I was mowing just fine stopped it and went to start it back and nothing the batter is OK, starter was engaging just will not do anything. I was thinking it might be a solenoid. But where is the solenoid to jump it??

The solenoid will be under the steering wheel cover, or near the battery if the battery is under the seat. Trace the positive battery cable and you will find the solenoid.
But I am confused , you say the starter is working? If so it is not the solenoid.
Provide more info.