Because you didn't gave the world's gate time to open
.Replace function main by this:
function main()
{
level_load("Test1.wmb"); //load the level
wait(4); // CRITICAL MASS =P
camera.z = 1000; //move the camera
camera.tilt = -90; //look "down"
}
EDIT:
Gamestudio does load the level, but pressing the button "u" doesn't create the model.
I was totally wrong then, let me take a closer look to your code.