Hi, your security key, is the Key, that was generated in the router when you were setting up the router for first use. First, you put in your user name & password, then you go to security settings and set your WEP/WAP Key, this is the security code that must be used to logon to that network from the WiFi.
Now if you either have an unsecured network, or an unknown WEP/WAP key, what you have to do is, Login to the Router with a browser, and get through to the security settings, and Generate a Key, now when you generate the key, copy/paste this Number to a notepad document, save it somewhere handy, say desktop. Now finish off with the router, save all the settings and reboot the router, now you are ready to logon to your wireless network, now startup the PC, and find the SSID, The name you gave your WiFi connection, now when you select, you will be asked for the WEP/WAP Key, open the notepad document and copy/paste that number you saved into the place provided, there, now you have your security set and be able to logon to your WiFi & router.
It is all also explained in your user manual, i have included a copy below.
http://www.retrevo.com/support/Netgear-DGN2000-Wireless-Routers-manual/id/23021ci772/t/2/