Could not load file or assembly 'DocumentFormat.OpenXml

I'm keep getting this error is VS2013

Could not load file or assembly 'DocumentFormat.OpenXml, Version=2.0.5022.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies.

In my web.config

<?xml version="1.0"?><!--  For more information on how to configure your ASP.NET application, please visit  http://go.microsoft.com/fwlink/?LinkId=169433  --><configuration><connectionStrings><add name="hjkom_med_web_udvConnectionString" connectionString="Data Source=M95;Initial Catalog=hjkom-med_web;Persist Security Info=True;User ID=HJkom-MED_web;Password=bvkeB7hh" providerName="System.Data.SqlClient"/></connectionStrings><system.web><compilation debug="true" targetFramework="4.0"><assemblies><add assembly="System.Data.Linq, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/><add assembly="WindowsBase, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>                **<add assembly="DocumentFormat.OpenXml, Version=2.0.5022.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>**</assemblies></compilation></system.web></configuration>

I have tried to insert windowsbase and a lot of other things.

I have search and search for a solution, do any body know how to fix it?