Select an Apple AirPort/AirPort Extreme Base Station (v3) (legacy) as the target host to create a monitor — the SNMP service should be up and running on it. Click New Monitor, then check SNMP Custom on the Favorites tab, click Next, and confirm the host. On the next page, click Select... to open the built-in SNMP MIB Browser and type sysConfUptime into the Find box to locate it in the OID tree, then select it and click OK. It reports the length of time, in seconds, the AirPort Base Station has been running. On the monitor's Main parameters page you can set the target's SNMP port (default 161), credentials, polling interval, and other settings — see the SNMP Monitor help for details. On the State conditions and Alerting tabs, configure when the monitor should change state and trigger an alert; since this is an INTEGER-type OID, Value bounds is the most useful condition here — trigger an alert if the value is unexpectedly low (e.g. under 10 minutes), since that indicates a recent, possibly unplanned, reboot or restart. Click Finish to create the monitor; you can adjust any parameter later.