Read original post by Michael Washington at LightSwitch Help Website
When you have Visual Studio LightSwitch 2010 (the released version) and Visual Studio LightSwitch 2011 (the Beta version) on the same machine, you may have a problem installing some extensions.
The problem is that it will only install in the Beta version of LightSwitch (Visual Studio LightSwitch 2011) not the released version (Visual Studio LightSwitch 2010).