I think you must find more information about how WLAN works and read some paper about IEEE 802.11x ...
If you are
not connect to the network I don't believe you can find private ip address of the router. And in fact even if you find it by any mean you can't connect (so attack) to it because you are not in the same network !
If you want the public address of the router, if it connect to a public network (ie Internet), try SE techniques to force the user to connect to one of your server.
But in fact I think you must learn how networks work before try hacking things or play with hacking tools
.
If you are
connected to the network so try that to find ip address ranges :
http://www.aircrack-ng.org/doku.php?id=find_ipor just a traceroute ... you will go through the router for join another network. Or go like a pig and "nmaped" all the ip range