I'm guessing you scroll the selector wheel to "Trim", then pick which channel to adjust. Typically, you set both to 0 for throttle and steering. Then get it as close as you can to where "center" is for steering and "idle" for the throttle linkage mechanically by adjusting the collars on the...
I didn't see you mention or see anyone else mention it, but first, remove the servo horn from the servo, turn your radio on and receiver on. Set your trim on your radio/xmitter to zero. The put the horn on the servo so that it's putting a bit of pressure via the spring on your throttle on the...