[LARTC] routing question

[routing <route@irax.com>]

So far I have been used to using linux to provide simple routing from my 
network to others using commands such as ip route add 192.168.1.0/24 via 
192.168.0.4 etc and it has all worked perfectly.
I also use smoothwall GPL to provice vpn services, however I have hit on 
a problem and am not at all clear on the way in which to proceed.
I now need to provide a route to services, the access to these is 
provided by a router on a network on the far end of a VPN. the  
computers on the  remote network can see the service I need to access, 
however when I try to provice a route to that system using a router on 
the remoted network by issuing a command such as 192.168.5.0/24 via 
192.168.15.6  in the router at 192.168.0.4 I get the following :-
RTNETLINK answers: Network is unreachable.

My question is , what way of providing access to this route do I need to 
follow, Is it GRE tunnels (not the best option as I don't have enough 
information on the remote router configurations and am not able to 
change their settings).  Do I have to use  new routing tables  or is 
there something else I must do to get this working?
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