[Vpn-help] trying to connect VPN to Zyxel 2602hwl adsl router

[Paul Webster]

I've configured a VPN from client v2.1.4 to a Zyxel ADSL router and the 
tunnel is active, but for some reason the router can't seem to find a 
route back across theVPN.

1. If I ping from the client (10.1.1.15)  to the remote office(e.g. 
192.168.1.2) I see packets on the remote lan but the return packets only 
seem to go as far as the Zyxel router (192.168.1.1) then disappear.
2. If I try to ping from inside the Zyxel router to the VPN 
client(10.1.1.15) when the tunnel is active it tells me the destination 
is not reachable.
3. The Shrew Soft VPN Trace only ever shows outbound traffic on the 
Security Associations page.

Why can't the router find it's way back across the VPN?

NETWORK TOPOLOGY:

[Shrew VPN client on XP 10.1.1.15]
    |
[netcomm nb9w ADSL router 10.1.1.1]
    |
(internet)
    |
[VPN Server Zyxel 192.168.1.1]
    |
(remote office LAN 192.168.1.x)