The security key is built in to the router, i.e. it was either set by your internet provider or by you when you first got it. If your ISP set it, they will have provided you with that information, either in the router box or in a separate letter. It won't be written on the router for security reasons. Check the box if you still have it, or anywhere you might have written it down.
It might be useful to tell us which ISP you use, and which make of router you have so that anyone with that particular model should be able to tell you how to access your router's settings directly. For example, to access my router's settings I go to
http://192.168.0.1, so give that a try and you may be able to change it from there.