Read this first: What's the basic idea behind the catapult?

You can control the length of the catapult arm with the "catapult controls"
(not the loader controls).

There are three different positions -- I'll call them "near" (for the
shortest arm length), "middle" (for the medium arm length), and "far"
(longest arm).

The catapult can only be loaded when it's in the middle position.  If you
want to put something or someone on top of the catapult, you first need to
use the controls to get the arm into that position.

If you want to put a rock on the catapult, use the loader controls.

You can only get either Ooya or Blount to get on the catapult.  Take a look
at how the loader controls operate to figure out what to do.