Referencing System.Web.Security.SingleSignOn

I'm walking through several examples on SSO with WIF and can't seem to get VS to recognize (or compile) any code that references System.Web.Security.SingleSignOn and can't add it by reference. Any ideas why or how to resolve this issue?
My web.config looks like this:
<?xml version="1.0"?> <configuration> <configSections> <sectionGroup name="system.web"> <section name="websso" type="System.Web.Security.SingleSignOn.WebSsoConfigurationHandler, System.Web.Security.SingleSignOn, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35, Custom=null" /> </sectionGroup> </configSections> <system.web> <compilation debug="true" targetFramework="4.0"> <!-- pull in ADFS assemblies --> <assemblies> <add assembly="System.Web.Security.SingleSignOn ...