First, I think you must read the documentation, because the configuration si completly different. To quick answer your qustion:
You must add on your server-root/msg-instance/imta/config/mappings, after the section INTERNAL_IP, a line like
$(192.168.0.0/24) $Y
and the execute a imsimta refresh. That will open the relay of the server to all the 192.168 networks.
Regards