Why can't I create a UI for my Add-In in Visual Studio 2010 Ultimate?

I don't know why the below checkbox is greyed out, I have general development settings and VS Ultim开发者_如何学JAVAate as shown.

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Does your add-in project name include a period? This would be sufficient to disable the tools menu option in the wizard. (See the note at step 4 at http://msdn.microsoft.com/en-us/library/80493a3w.aspx for details.)