the type expression is defined in an assembly that is not referenced code example

Example: you must add a reference to assembly 'System.Core, Version=4.0.0.0, Culture=neutral

//place this in the web.config file
<add assembly="System.Core, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>

<add assembly="System.Data.DataSetExtensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>

<add assembly="System.Xml.Linq, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>