Code:

---

function main()
{
    fps_max = 50;
    level_load (intro_wmb); 
    sleep (5);
    time_smooth = 0.9; 
    	} // remove this bracket!
    	if(key_cuu == on && select >= 0){
    		select += 1;
      }
      if(key_cud == on && select <= 2){
    		select -= 1;
      }
      selection();
} 

---

The ifs in the main function do not make much sense.
It seems there should be a while-loop around them.

edit:
Even better would be two key functions:
Code:

---

function menu_selectionUp()
{
  select = cycle(select+1,0,2);
  selection();
}

function menu_selectionDown()
{
  select = cycle(select-1,0,2);
  selection();
}

function main()
{
    fps_max = 50;
    level_load (intro_wmb); 
    sleep (5);
    time_smooth = 0.9; 

    on_cuu = menu_selectionUp;
    on_cud = menu_selectionDown;
}

---