First, if you want to disable when entering a trigger, you should probably be using
function OnTriggerEnter(other : Collider)
{
...
}
Second, ensure that this trigger script is on the same object that you are expecting to have the MouseLook component. Your code right now is assuming this. If the MouseLook is on the colliding object you would need to do this
function OnTriggerEnter(other : Collider)
{
var mouseLook = other.GetComponent("MouseLook") as MouseLook;
if(mouseLook != null) {
mouseLook.enabled = false;
}
}