In the forums, Craig Lee responds to the  problem reported in this thread:

We are planning on releasing an FAQ of known issues, but here is a preview to see if this helps you.

Issue
Visual Studio displays a Package Load Failure error message for Package 'Microsoft.Data.Entity.Design.Package.MicrosoftDataEntityDesignPackage’ when you double-click on a .edmx file

Workaround

This issue is caused by earlier installations of the MCrit June CTP.

1.    If it is installed, uninstall the ADO.NET Entity Framework Tools CTP from “Add/Remove Programs” in control panel

2.    Use gacutil.exe to verify that the following DLLs are not in the GAC. 

a.    Microsoft.Data.Entity.Design.dll

b.    Microsoft.Data.Entity.Design.EntityDesigner.dll

c.    Microsoft.Data.Entity.Design.Package.dll

d.    Microsoft.Data.Tools.XmlDesignerBase.dll

3.    Use Regedit.exe to check for and remove the CodeBase value from the registry at:

a.    HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\VisualStudio\9.0\Packages\{8889E051-B7F9-4781-BB33-2A36A9BDB3A5}

4.    Reinstall the the ADO.NET Entity Framework Tools CTP