Might be the same issue as http://internalsviewer.codeplex.com/WorkItem/View.aspx?WorkItemId=5882. Make sure you download the correct version - for either SSMS 2005 or 2008.
Download choice page:http://internalsviewer.codeplex.com/releases/view/21139
Paul White
SQLPerformance.com
SQLkiwi blog
@SQL_Kiwi