I have used ZyWALL a few years ago. It was very simple...
The only things required were activating the VPN module, specifying local and remote address ranges, and specifying a key.
I used SSH Sentinel to connect to the remote private network.
If you use SSH Sentinel, you may have to click on Acquire virtual IP
(or something like that) to be assigned an IP from the range of the remote private network (due to the settings of said private network).
It might be a useless question but can those two machines connect to each other?
Edit - make absolutely sure your subnet mapping is correct - example:
ZyWALL A - Local:
Address Type = subnet
IP Addr Start = 192.168.10.0
ZyWALL A - Remote:
Address Type = subnet
IP Start = 192.168.20.0
ZyWALL B - Local:
Address Type = subnet
IP Start = 192.168.20.0
ZyWALL B - Remote:
Address Type = subnet
IP Start = 192.168.10.0
This will map the subnets so the computers can see each other.
Of course, you NEED to configure your local computers properly - the network configuration of the remote computer should be done in a way, that if you brought said computer over to the local network, it would be accessible.
So configure your network masks, firewalls and routers on servers and client computers so they can communicate.
ZyWALL configuration is so extremely simple that even a basic user should be able to configure it properly. The rest of the network is usually the problem, not the ZyWALL config.