[Moo] Tween not a constructor

[cogear]

Everytime I try to use mootools I have trouble. Please help! I am
obviously missing something. Trying to keep it simple here is my code
so far.

<script type="text/javascript">
window.addEvent('domready',function() {
        var otherEl = $('dropdown');
        console.log(otherEl);
        var myEffect = new Fx.Tween(otherEl, {transition:
Fx.Transitions.Quint.easeInOut});
        });
 </script>

This throws the error:
Fx.Tween is not a constructor

Can anyone please tell me what I am doing wrong?

Thanks