[Spinner]

As several others have said, the throttle needs to be set up in the "Controls" options.
After you click on the "Controls" button on the main game screen, scroll down to the "Hotas" section at the bottom. Select the empty box accross form "Power" and then move your throttle. An entry will be made in the box.
That should fix it if that is all that was wrong. If your throttle works in reverse, go back and do it again but move the throttle the other way.