I'm having a little trouble picturing your topology. I'm assuming the Cisco you mention is the termination point for the VPN. And that would be on your SSG LAN somewhere. But what are the connections?
To create the route we need to know the interface the traffic will leave the SSG on and the gateway ip address that will receive the subnet (the cisco I assume?). Substitute the correct interface and the ip address of the vpn box reachable via that interface as the gateway.
set route 192.168.5.0/24 interface ethernet0/0 gateway 1.2.3.4