Hi,
The issue you are reporting sounds like you have not centered the roll trim. I assigned my top left hat switch to pitch trim up/down and roll trim right/left, which enables me to control the plane quite well. If after you assign your joystick's buttons the aircraft still veers to the left when you roll down the runway, it is likely two buttons are still controlling the roll trim, besides those you have just set, and you have to go into your joystick calibration and find those two extra buttons or switches that are assigned by default, and exclude them. They are likely interfering with your settings, and are likely not centered. Make sure there are no other buttons, switches or dials that are linked to the roll trim, and if there are, disable them. I had the same issue and found out two dials were set for roll trim by default, and they were not centered, which caused my Cessna to steer all the way to the left and up, crashing after a few feet. Hope that helps...