From LedHed's Wiki
How to setup a Site to Site IPSEC VPN between Smoothwall and pfSense.
Smoothwall Express
Go to VPN -> Connections
Left: RED IP
Left Subnet: GREEN Subnet
Right: pfSense WAN IP
Right Subnet: pfSense LAN Subnet
pfSense
Go to VPN -> IPSEC
pfSense is a but more straight forward:
Remote Gateway: Smoothwall RED IP
Remote Subnet: Smoothwall's Green Subnet