Hi Clement,
See Tomas' response here:
https://supportforums.cisco.com/discussion/13238976/snmp-traps-apic
This feature may be version dependent. I have a simulator currently on 2.0(1p) and I did not see the Monitoring Objects option. What version are you running? I'd be very interested to see if you can see it in your version.
Its also possible i'm looking in the wrong place:
Fabric >Fabric Policies > Monitoring Policies > Common Policy ? Callhome/SNMP/Syslog > SNMP Tab