The name 'SPClaimsUtility' does not exist in the current context while trying to create custom login page.

I am unable to resolve 'SPClaimsUtility' even though I added Microsoft.SharePoint.IdentityModel .dll reference. I added the .dll from\assembly\GAC_MSIL\Microsoft.SharePoint.IdentityModel\3.5.0.0__31bf3856ad364e35. I am not able to find the .dll anywhere else. Appreciate your help. Thanks.