Problem>> The type or namespace 'Linq ' dose not exists in the namespace 'System' ('are you missing an assembly reference ?)
Solution >> goto web config and all system .core to it .System.linq exists in System.core assembly so add it to config file.
This is how it looks like
 <compilation debug="false" targetFramework="4.0">
      <assemblies>
        <add assembly="System.Data.Linq, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
 <add assembly="System.Core, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
      </assemblies>
    </compilation>