If by security key you mean the WiFi Key - they all vary, even between the exact same model.
You will have to connect with an Ethernet cable. Make sure you don't have a static IP then open up a command prompt and type:
ipconfig
The IP address of your default gateway will be the IP address of your router (usually 192.168.x.x)
Type this address into a web browser. You will most likely be prompted for a username and password which can be found using this website - http://portforward.com/default_username_password/
After this you should have access to the routers web management page. The wireless key will be under something like a 'Wireless' or 'Security' tab.