cant conect to the L2TP IPSEC VPN conection

Hello.

i Tried to establish IPSEC VPN Server in microsoft windows server 2008 acording to this guide:SSTP Remote Access Step-by-Step Guide: Deployment

i used the windows XP SP3 as client. when i using PPTP protocol for my VPN connection every thing works fine.

but when i change the VPN type to L2TP/IPSEV VPN this error show up:

1- am i did right for creating IPSEC VPN connection or the guide only work for sstp connection?(if i used wrong guide please tell me how can create IPSEC/L2TP VPN conection in windows server 2008)

2- i downloaded certificate and install it on my client but VPN connection cant access to it. how can add the certificate to VPN connection?

Thanks.