mercury 9.9 wont't lock in reverse

when i put my outboard in reverse it won't lock an wants to raise up how do i lock it and what do i adjust i believe it's suppose to lock when i put it in reverse because i've looked all over for a lock lever and can't find one and there is a flywheel lock so that it can't be started in gear is that the same lock but for reverse too!!!!!! oh by the way it's a 2004

 

There is generally an large pin located on the part of the mount that attaches to the transom. There are series of holes that the pin can be placed in to adjust the angle of the motor shaft relative to the transom. The anti kick back lock usually catches on this pin. Make sure you have the pin, and make sure when fully down the anti kick back hooks will engage the pin.