1) Change your .net profile from Client profile to to .Net Framework 4.0
http://msdn.microsoft.com/en-us/library/bb398202.aspx
2) Check your Embed Interop Types flag
http://weblogs.asp.net/cazzu/archive/2011/03/11/check-your-embed-interop-types-flag-when-doing-visual-studio-extensibility-work.aspx