Commented Unassigned: Could not load file or assembly 'DocumentFormat.OpenXml, Version=2.0.5022.0 [9142]

i am getting the following error, can someone guide me what i am missing
Could not load file or assembly 'DocumentFormat.OpenXml, Version=2.0.5022.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The system cannot find the file specified.":"DocumentFormat.OpenXml, Version=2.0.5022.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
Comments: ** Comment from web user: roberttanenbaum **

Go to the first page of the documentation and download the DocumentFormat.OpenXml.dll that is there and add it to your project assembly.

Requirements:

To use ClosedXML you must reference the following DLL: DocumentFormat.OpenXml.dll (part of MS Open XML SDK 2.0)

Note that if your are deploying this to a server, make sure you deploy the .dll along with your application.