Adding air molecules increases the pressure because there is more in the balloon. If it were water, imagine putting more water in the balloon. That might be easier to imagine.
But air isn’t really fixed in its shape in the same way as water is it?
Well, the molecules in air move around lots and they bump into each other. They also bump into the inside of the balloon, which is how it puts pressure on the balloon.if you put more air in, you put more molecules in, so there is more bumping around – so more pressure!