which assembly is used in .net 3.5 to used entity framework?

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.NET3.5

.NET

In my project i used entity framework but when i run the application it show me error that The type or namespace 'NothwindEntities'name could not be found (are you missing a using directive or an assembly reference?).
i used assembly in web.config file

<add assembly="System.Data.Entity, Version=3.5.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"/>

Please help me to solved my error.

Regards,
Manohar Khillare