Pual, I got frustrated with IPSEC VPN addon not working with 5.6. I ended up installing IPCop servers which are firewall/IPSEC routers. IPCop is designed to use IPSEC and it works immediately after install. IPCop also has a very nice web interface for administration.
Basically, I have decided to only use stock functions of SME (except port forwarding). Trying to get new releases of SME to work with addon functions was to much headache. Using SME with IPCop is a very effective solution that works using default installs of each.
IPCop allows for a third nic install for a DMZ. If you only have a single internet IP address available, you can use the DMZ as your external subnet for SME. Or SME can be set up as a standalone server (one nic). IPCop has a very nice port forwarding setup. IPCop 1.3 can forward PPTP connections to your SME server without having to deal with iptables (in this case, your SME has to be connected to the DMZ subnet).
You should look into IPCop. Another option is the cable/dsl firewall IPSEC vpn router from linksys. It can handle up to 70 ipsec tunnels and is cheap and easy to use. I actually use the linksys ipsec vpn routers to connect remote locations to my central IPCop server.
good luck,
ryan