There are two possible reasons for not being able to find an OID in the MIB tree:
OID number or name is mistyped
MIB containing the OID is not in the application database
Double check the OID number or name first.
If this is OK, then you need to add a MIB containing the OID to the application. Download the MIB (from vendor website for instance) and then add it to the database by going to > Settings > MIB Settings > Modules.