near the front of ur controler on the top there is 2 rounded buttons. those r ur steering trim. this adjusts wether it goes straight or not. they also control how far each wheel turns.
what u want to do is get something to prop ur chassis on so the wheels hang off. then turn on ur xmod and the controler. there is an L and an R above each button. the L is Left. the R is Right. put the xmod on the block and adjust the ST. TRIM until the wheels both go straight. then put it back on the ground and turn a few times.

if that doesn't work then on the back of the controler is the steering range. it is generally used for how far u can be from ur RC car. on the xmods it is used for how sharp a corner to be taken. really sharp turns u want it on max. for shallow turns u want it on min. try playing with this until it turns the same for both directions.