L2TP VPN on ZyWALL USG 300

I am trying to create a VPN so that users can connect to shared folders on my Windows Server Active Directory domain on my LAN. I am very unexperienced in VPN's, so I followed the steps in the User Guide in section 4.6 (L2TP VPN with Android, iOS and Windows), pages 85 thru 101, but I am unable to get it working.

I was wondering if there are some better "complete idiot" steps somewhere.

Any comments appreciated.

Thanks.