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.