Hey guys,
I've also fixed this issue, wooo hooo
The right thing to do is, in the "Policy" tab, we do not select "Obtain Topology Automatically or Tunnel All".
Afterwards we need to install two manual routes.
type: include, address: 0.0.0.0 netmask: 0.0.0.0
type: exclude, addresse: <your_monowall_public_address>, netmask: 255.255.255.255
With these two rules, all traffic will be redirected through the VPN tunnel, except for the public addresse of Monowall, this seems to be required to not break the tunnel.
Of course, this topic is also related:
http://forum.m0n0.ch/index.php/topic,4601.0.html#msg14130(If the ADSL router is in face of Monowall, the ADSL router must contain a route for the private adresse of the IPSec client, to fix the reverse path.).
Hope this helps.