I'm not with Laminar Research, just a fellow simmer in the UK.
Are you referring to "flying straight" during actual flight, or when you are on your take off roll down the runway?
If the former, then firstly re-calibrate your flight controls i.e. yoke, rudder etc., just to make sure that everything is set up properly.
Once the aircraft is in flight, then you have the following trim options:-
Yaw trim right: keyboard button "7"
Yaw trim left: keyboard button "5"
Elevator pitch trim down: keyboard button "["
Elevator pitch trim up: keyboard button "]"
Aileron roll trim right; keyboard button "0"
Aileron roll trim left: keyboard button "8"
If the problem is related to the latter issue, then it is quite normal for the Cessna Skyhawk to veer left on the runway during the initial part of the take off roll, which I believe is due to a torque reaction from the propeller. In this case, you will always have to apply right rudder to maintain a straight trajectory down the runway.
Hope this helps