Although this questions is nearly three years old, the Airport Utility limitations still exist. I did not want to roll back to 5.6, so I searched for another solution, which I found.
These are the basic steps:
- In Airport Utility, select the base station to configure, and click edit.
- Then go to File, and export the configuration file.
- Open the configuration file in a text editor, and search for the key slCl.
- Edit the string field to match the IP address of the system logger.
- You can also set the log level by adjusting the string for key slvl.
- Finally, go to File again and import the configuration file to update the settings.
Kudos to Frits Hoogland and his blog "How to make apple wifi routers do remote logging".1
Related to this, please see my post on the details actually making it into the log: Airport Extreme firewall logging