As this is only a beta there are no binaries, you will need to grab the source yourself and compile.
Replacing the default encoders in .NET 4.0 will require you to use the DLL from the Net4 project and to make a web.config change to the httpRuntime node as follows

<httpRuntime encoderType="Microsoft.Security.Application.AntiXssEncoderAntiXssEncoder, AntiXssLibrary.dll"/> 

The runtime encoder feature should work with both WebForms and MVC (either the webforms or Razor view engines).

If you want detail, please go to the Source.
Source: http://idunno.org/archive/2011/04/23/antixss-4-1-beta-1.aspx