“click here to reload the designer” message when creating new SL application in Visual Studio 2010

Generally Visual Studio 2010 doesn't come with Silverlight 4 . They have Silverlight 3 capability only.
If you want to develop Silverlight 4 app in any of the above two IDEs, you need to install additional add-on that can be installed from http://www.microsoft.com/downloads/en/details.aspx?FamilyID=b3deb194-ca86-4fb6-a716-b67c2604a139 (34 MB).
Once you have installed, you should start developing Silverlight 4 app.

But when you get the error message “click here to reload the designer” message with the Details pane referring to a null reference exception "object reference not set to an instance of an object"
Just uninstall all the Silver Light named programs from Control Panel.
And follow the process mentioned above .
Don't try to install SilverlightDeveloper.exe before installing SilverlightTools.exe because results in the above error.

Hope this helps!