You're lucky, I've a little free time today.
Here is the code :
Code:

---

var speed = 5;   // play with 5
var worldCentre;

function main()
{
   level_load("yourLevel.wmb");
   wait(2);

   vec_set(worldCentre,vector(0,0,0));
   mouse_mode = 1;
	
   while(1)
   {
	mouse_pos.x = pointer.x;
	mouse_pos.y = pointer.y;
		
	vec_to_angle(camera.pan,vec_diff(temp,worldCentre,camera.x));
		
	// zoom in/out
	camera.arc += mickey.z * speed * time_step;
	camera.arc = clamp(camera.arc,10,100);
		
	// pan left/right
	if (mouse_right)
	{
	   vec_rotate(camera.x,vector(mouse_force.x * speed,0,0));
	   vec_add(camera.x,worldCentre);
	}
		
	// Orbit? don't know how do you achieve this,
        // so this part maybe not exactly what you want.
	// The camera will orbit around the world center vertically.
        if (mouse_middle)
	{
	   vec_rotate(camera.x,vector(0,speed * time_step,0));
	   vec_add(camera.x,worldCentre);
	}
		
	wait(1);
   }
}

---

Hope this helps!