Try adding the proper
System.Data.Entity
assembly to your web.config with regards to your framework.
Such as for 4.0:
<compilation debug="true" targetFramework="4.0">
<assemblies>
<add assembly="System.Data.Entity, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" />
</assemblies>
</compilation>
Good luck,
OI