When you do a MIB walk you expect to get ~15000 OIDs and it will have the SPU CPU usage as ".1.3.6.1.4.1.2636.3.39.1.12.1.1.1.4" or "jnxJsSPUMonitoringCPUUsage".
You may have to wait till the SNMP MIB walk complete, which will take few minutes and it has to print ~15k lines.