On page 5-16 look at figure 5-6
Go to that screen on the router. From the router admin homepage, on the left column under "Advanced" click on "Port Forwarding/Port Triggering".
Click on the button at the bottom "Add Custom Service"
Enter:
Service: Subsonic
Protocol: Select TCP (you do not need UDP for SS)
Enter the same port number into the starting and ending port # blocks - the port number that Subsonic is listening to. (Default on Windows is 4040)
Server ip address: Enter the INTERNAL ip address of the host that has Subsonic installed. (192.168.1.X X = where your server is, if you don't know, open a command prompt on the server and type ipconfig /all)
Click Apply
Test using canyouseeme.org to check that the port is open and responding from the internet.
If you are going to use SSL then you'll have do do the same thing for that port - 4443 is the SS default for Windows. I'd try to get it running without the SSL first - then if it works add in SSL later if you want to use it.
Enjoy Subsonic
Listen to some relaxing tunes to reward youself for all your hard work.
Let me know if you need anything else.
