Think you set "Throttle 1" in your controls to whatever mouse wheel is in-game...unsure as I haven't played with a mouse in many years.
I use my mouse wheel for throttle. IIRC select control mode 1. Then select mouse as controller. Then set the wheel as throttle. You can set whatever else you want as the rest of your controllers (I've got my joystick for everything else). Then you have to calibrate the mouse wheel. I use a standard MS optical mouse and the wheel "clicks" as it rolls. To calibrate I roll five clicks up then ten clicks back then five up. You can make that as fine or as course as you want. I find 10% increments work pretty well.When you're actually using the mouse you can roll past the "stop" and wherever you end up just becomes the new "stop".Hope that helps.