Project unavailable if Solution Contain Multiple Projects. [VS2010]

When I am downloading sample projects from various websites.

And if it contain multiple project in One solution.

And when i clicking on the main solution file……it opens only one main projects other projects remain unavailable though it is present in folders…with their project files………

and though i am trying to add manually each project in Solution though it gives me errore see image of error message also....

NOTE: i am using VS2010 and i am trying to load SilverLig开发者_如何学运维ht Projects

See below image

error when manually adding project in solution

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Maybe this link will help you

http://forums.silverlight.net/forums/p/54197/386127.aspx#386127

It seems maybe SDK is not installed or badly installed..