The first thing you should do when you see a js error like this is delete the cache file, here is a link to instructions on this:
common_installation_issues.pdf
If this does not resolve the error then you may have a corrupt configuration folder, if this is the case you will need to take the steps listed here to resolve the issue:
showpost.php?p=12293&postcount=2
Please post back and let us know if the issue persists.