You do not need any drivers for a joystick connected to the USB port. Also, you do not need to assign "Throttle 1" and "Throttle 2".
If you only assign "Throttle 1", all the engines of a multi-engine plane will be slaved to the one throttle. If you assign "Throttle 1" and "Throttle 2", then you will have to control the throttle for the left engine(s) and the right engine(s) in a multi-engine fighter.
It is not unusual to have to map the "Throttle 1" command to the input of the stick which would control the throttle. As you can see, no joystick manufacturer actually calls anything a throttle on the stick. Finding the throttle on the stick just requires you to move the throttle and find the numbers in parantheses (next to each axis) which corresponds to your motion.
Then assign "Throttle 1" to that input. Click on the axis (or double-click), press "Modify" if the command list does nt show up, scroll down to find "Throttle 1", select it with the mouse, then hit "OK". Hit "OK" at the "Map Controllers" panel, and as long as the stick has been properly calibrated, it will work.