Try the following:
(if you don't know the password try admin)
Under Advanced Setup go to Virtual Server.
Set the Private IP to that of the machine running DC (if you are unsure go to the machine running DC then: Start > Run > Type: cmd > Type: ipconfig (it should be something like 192.168.xxx.xxx))
Enter the same value for Private Port and Public Port (ex. 5423)
You'll need to enable both TCP and UDP so it may need two entries.
Then in DC you'll need to specify the port (same port you used above (ex. 5423)). From the DC FAQ:
"Q: I have a internet connection sharing device and would like to set Direct Connect up for active mode. What ports does it use?
A: By default, Direct Connect uses random ports for TCP and UDP data. This can be configured in the Direct Connect preferences. In the preferences window, choose ‘Connection’ and click the ‘Advanced…’ button. A new dialog will appear. From here you’ll be able to configure Direct Connect’s port usage."
Give that a try and see if that helps. Unfortunately I do not use DC but this should work.
P.S. Router documentation can be found here: http://www.speedstream.com/support/pdf/2602_manual.pdf