|Ok, you're going to have to either add the route to the Dlink (it's the network appliance between the two networks), or as I said above, put it in the same network.|
I would recommend the same network. There are only two reasons I can think off of the top of my head for adding a second, separate, network to your LAN:
2) you've run out of IP's in the first network.
Since neither seems to apply to you, plug the uplink cable from the ASA into a LAN port on the Dlink. Give the Dlink a LAN IP in the same subnet as your ASA using the ASA's LAN facing IP as it's default gateway address.
If you already have a DHCP server running, disable the DHCP service on the Dlink.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.