Don't know nuffink about DC++, but port forwarding is where the incoming data needs to know where to go. Without a router, it goes to the connected PC. With a router, it needs to know WHICH PC to go to. In the router settings, there will be an option to specify, for any IP port, which PC (IP address) the data should go to. If you know which port (or ports) DC++ uses, and the IP address of the PC you want the data to go to, Bob's your Uncle.
(May not be called 'port forwarding'. On my - Belkin - router, the option is called .Virtual Servers

.