wifi.log disappeared?
Mountain Lion does log a limited amount of information to 

/private/var/log/wifi.log

To read that file – and its siblings such as wifi.log.0.bz2 – use Console. 

Included with the operating system is a diagnostics app that can enable debug logging. With that level of logging enabled you can then use Console to: 


view all messages
add a system log query
and so on.


Also in Stack Exchange: 


Wifi Diagnostics get WiFi performance (2013-01-02)