The wheels will go below the runway if the visual scale is set too big for the plane's listed gear dimensions. It's easier to demonstrate than explain . Edit that caribou and change the visual scale to something like five (on the Plane Setup tab), you'll see the problem get worse. This means the author of that plane didn't bother to reset the height of the landing gear and just gave you a bigger look. Compare the gear height (that's on the fuselage tab) to that of the normal Cessna and you'll probably see they're the same.

At any rate, just make those numbers bigger (more negative) until your plane sits correctly on the runway. You can actually make the thing float in mid air if you wanted to. Again, you're looking to modify the Main Gear Location's "Up" value and the Steering Gear Location's "Up" value. It's hard to explain...just edit the thing and I think you'll see what I mean.

Chuck