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.
For more info on adding a MIB, go to article Configuring MIB Modules.