The debug capabilities in Internet Explorer 6 or higher are disabled by default but can be turned on by going to Tools Internet Options Advanced. The Advanced tab defines two items that need to be unchecked (a check means you can't debug):
Disable script debugging (Internet Explorer)
Disable script debugging (Other)
Also, check the Display a notification about every script error checkbox below these items so that you're notified as errors occur in a page. If this box isn't checked, an icon appears in the lower-left corner of the browser; unless you click it, you may miss noting that a client-side error occurred. A side effect of checking this checkbox is that ...