There are two possible solutions: There was a conflict with an older version of Scriptaculous and jQuery (Scriptaculous was attempting to extend the native Array prototype incorrectly) - first try upgrading your copy of Scriptaculous.
If that does not work you will need to use
noConflict() (as alluded to above). However, there's a catch. Since you're including a plugin you'll need to do the includes in a specific order, for example:
Hope this helps to clarify the situation.