Several people have already written about how to add Intellisense to Visual Studio for SharePoint 2010’s Client Object Model (ECMAScript or JavaScript). Here’s one that I followed: http://praveenbattula.blogspot.com/2010/06/sharepoint-2010-ecmascript-intellisense.html
Following those instructions, mine still didn’t work. I finally got it to work after removing the <script> tags in my file.