Unable to add selected web part(s).

ERROR:
Assemblies that implement ASP.NET Web Parts and are installed into a partially trusted location, such as the bin directory, must be compiled with the AllowPartiallyTrustedCallersAttribute set for import to succeed.

SOLUTION:
Add [assembly:AllowPartiallyTrustedCallers] to AssemblyInfo.cs

REFER :
http://msdn.microsoft.com/en-us/library/system.security.allowpartiallytrustedcallersattribute(VS.71).aspx