I haven't downloaded the source, but when I try the jump code, it doesn't work.
I ended up doing this to get it to work:
if (Ax.keys.pressed(AxKey.SPACE)&& this.wasTouching(DOWN)) {
velocity.y = -360;
y -= 1;
}
I had to use wasTouching because isTouching returned false, and I had to raise the y one pixel or it wouldn't budge.