yeah, like domination said, check the linkage of the servo. if it is new like I'm guessing it may be. the servo may just be out of alignment. so do it like this.
1- with electronics all off. take horn off servo (while its of check the splines on the servo adapter and inside the horn.. just in case. never hurts to check)
2- turn radio and reciever on
3- adjust steering trim on radio to centered.
4- turn electronics back off
5- straighten front wheels as much as possible
6 - re attatch horn (making sure to keep wheels pointing straight
7- turn on radio and electronics
8- attempt to turn right and left
9 - fine tune the steering with your adjustment on the radio to reach full center
if this is all done, and it still doesn't turn, your servo may be mangled inside, and its time to invest in one with metal gears (stadium truck steering takes a beating. especially if you're bashing)
p.s. I'm not sayin all this to be a jerk or anything , I'm just laying it out easy because maybe someone doesn't know.