To repeat what Jason Salaz wrote: if you want to change SNMP settings you probably have to install Airport Utility 5.6 alongside 6.x. Trying to replace 6.x doesn't work well... either the install fails (typically) or if you get it to install, it will get overwritten later by an update.
If an automated script doesn't work, you can extract the app from the 5.6 .pkg file, available from Apple downloads, using a tool like Pacifist. Another option is to simply copy the app from a machine that is running it, rename it to Airport Utility 5.6.app, and then copying that to your Applications folder. Then you will be able to use both 6.x and 5.6.
To get to the (rudimentary) SNMP settings, open Airport Utility 5.6, click Advanced, then the "Logging & Statistics" button. In that panel you will find settings for turning on/off SNMP, allowing SNMP over internet (WAN), and the SNMP community string. All settings are changeable.