If it is linked to snmpdv3ne or snmpdv3e, then the relevant config file is /etc/snmpdv3.conf, not /etc/snmpd.conf.  (The active snmp agent is controlled by snmpv3_ssw. snmpv3_ssw -n will switch to the non-encrypted, v3 agent.)
To allow AIX monitoring, you just need to ensure that the config file (snmpd.conf or /etc/snmpdv3.conf) allows the LogicMontor collector to query the system.  
Ensure that the collector has access with the community you expect, and that all views are included.
  Ensure that any line such as:
 
VACM_VIEW defaultView 1.3.6.1.4.1.2.6.191 - excluded -
is changed to included:
VACM_VIEW defaultView 1.3.6.1.4.1.2.6.191 - included -
Then restart all the snmp services:
stopsrc -s snmpd;stopsrc -s dpid2;stopsrc -s aixmibd;stopsrc -s hostmibd;stopsrc -s snmpmibd
startsrc -s snmpd;startsrc -s dpid2;startsrc -s aixmibd;startsrc -s hostmibd;startsrc -s snmpmibd
If you are not receiving data for CPU, memory, etc:
Ensure hostmibd, snmpmibd and aixmibd are not commented out in /etc/rc.tcpip
Ensure that snmp configuration allows connections from 127.0.0.1 with public community (or the appropriate community if you’ve changed that used by aixmibd away from the default.) (Note: the subdaemon’s actually connect to hostname “loopback” – so make sure that correctly resolves to 127.0.0.1)
Ensure aixmibd is running: lssrc -s aixmibd
Check /var/tmp/aixmibd.log to see whether aixmibd has correctly registered with the snmpd system, and for clues as to why not.
Sample /etc/snmpdv3.conf:
VACM_GROUP group1 SNMPv1  public  -
#VACM_VIEW defaultView       internet                   - included - 
VACM_VIEW defaultView        1.3.6.1.4.1.2.2.1.1.1.0    - included - 
VACM_VIEW defaultView        1.3.6.1.4.1.2.6.191.1.6    - included -
# include normally excluded snmpv3 related MIBs from the default view
VACM_VIEW defaultView        snmpModules                - included - 
VACM_VIEW defaultView        1.3.6.1.6.3.1.1.4          - included - 
VACM_VIEW defaultView        1.3.6.1.6.3.1.1.5          - included -
# include normally excluded aixmibd managed MIBs from the default view 
VACM_VIEW defaultView        1.3.6.1.4.1.2.6.191        - included - 
VACM_VIEW defaultView 1.3.6.1.2.1.25.2.3.1 - included - 
VACM_VIEW defaultView 1.3.6.1.2.1.25.2.3.1.3 - included - 
VACM_VIEW defaultView 1.3.6.1.2.1 - included - 
VACM_VIEW defaultView 1.3.6.1.4.1.2021 - included -
VACM_ACCESS  group1 - - noAuthNoPriv SNMPv1  defaultView - defaultView -
NOTIFY notify1 traptag trap -
TARGET_ADDRESS Target1 UDP 127.0.0.1       traptag trapparms1 - - -
TARGET_PARAMETERS trapparms1 SNMPv1  SNMPv1  public  noAuthNoPriv -
COMMUNITY public    public     noAuthNoPriv 0.0.0.0 0.0.0.0 - 
DEFAULT_SECURITY no-access - -
logging         file=/usr/tmp/snmpdv3.log       enabled 
logging         size=100000                     level=0
smux            1.3.6.1.4.1.2.3.1.2.1.2         gated_password  # gated 
VACM_GROUP director_group SNMPv2c public  -
VACM_ACCESS director_group - - noAuthNoPriv SNMPv2c defaultView - defaultView -