That page is similar to mine, so I would bet you click on the radio button "Enabled" to enable port forwarding (a.k.a. virtual server, I guess).
The fields on the top let you create new virtual servers (i.e., new firewall rules). You'll need to add (at least) one for CS.
The Name is just any name you want to appear in that list you see at the bottom. It's there so you'll later know why you added it. Try something like "CS Server Setting 1" for that.
Private IP is the IP address of the machine you are hosting the server on. That's the 192.168.x.x address you have.
Protocol is TCP, UDP, or Both - at least on mine. CS uses UDP, but you can try Both is DLink has that.
Private port is the port number you run CS at on your internal server machine. 27015 is fine.
Public port is the port you want the rest of the world to contact your CS server on. 27015 is probably expected.
Click the radio button "Always" unless you want to set up a schedule.
Press Apply to add the new rule to the list.
You might have to check it to turn the rule on or maybe that's done automatically when you hit Apply.
For CS, I think you might need to port forward 27020, too as well as 27015. It seems that I had to do that. Just add a second rule like "CS Server Setting 2" and enable it.
Browse to
WhatIsMyIP to find out what your pubic IP address is. If you are using dynamic IP addressing with your ISP, which I think you are, that will change from time to time. This is the address that you tell your friends to connect to.
If DJ-Chris has a DLink router, he can probably give you more specifics. What's above should be close.