There is a conflict since you are already using jQuery on your page. Check out this thread for more info: showthread.php?t=8878
Basically, edit the gallery.js file for your selected gallery design and change any instances of $( to document.id( - i think there will be at least 3-4 places. If you are using any other gallery types, make sure to update those js files too. jQuery uses its own $ functionality, so changing the code will allow MooTools to work properly. I have this issue logged so that we are aware of it for future releases.