As many others who wanted to use this wonderful library i.e. Mootools, inluding it in wordpress was a pain as it conflict with others one.
Check if you use the library jQuery in your header.php of your wp theme:
then visit those pages:
To use your mootools library, you must for ex. insert this piece of code after jQuery and the other library have both loaded:
(for me it was in the header.php page)