Wow, if only it was that easy. I looked at my P-51 profile (for which the throttle worked) and there is nothing assigned to the throttle. In the controls section there are only key buttons and POV options.
I wish you luck David. I am getting frustrated trying to fix this small problem.
Update: Okay, I've fixed it. The throttle is an axis command. Didn't know that, but, now I do...