Hello,
Please use below link for configuring the Dial Up VPN.
https://kb.juniper.net/InfoCenter/index?page=content&id=KB15272&actp=search
And please use below link (Chapter 5, search for 'Example: XAuth Authentication (External User Group)') for configuring Radius for Xauth.
http://www.juniper.net/techpubs/software/screenos/screenos6.3.0/630_ce_UserAuth.pdf
Regards,
Rushi