if you mean that its not staying in gear, say for example, you put the car in gear and you take off and it pops out of gear, chances are you have done what i did & pushed the gear out of position.
if this is the case, take it to someone as soon as you can, and try avoid using that gear, if it becomes unusable, cahnces are you'll need a new gearbox/get the old box reco'd just like i did.
trx boxes arent the cheapest things around. they're fairly difficult to find too.