I have a DTSX file in a project I\'m taking over. I have Visual Studio 2005 Pro, but it just opens it as an XML file. SQL Server Management Studio 2005 does the same.
You need the Business Intelligence Studio ..I've checked and my version of VS2008 Pro doesn't have them installed.
Have a look at this link:
http://www.microsoft.com/downloads/details.aspx?familyid=3C856B93-369F-4C6F-9357-C35384179543&displaylang=en