Hmm, either the walkthroughs are wrong, or you've misread them. It's not the dirt ramp above ground, but rather the ramp below ground, which leads to the "reactor chamber". Check out that you haven't missed an exit in the below-ground area. Possibly at the bottom of the screen.
Edit: By the way, this should have been posted in the hints forum.
Edit: By the way, this should have been posted in the hints forum.