Either that MIB is not populated or that device doesn't support that OID or you don't have SNMP setup correctly. It is not the same OID on every device.
The long answer...
If you have SNMP working you can right click on a device and use snmpwalk under the tools menu. When you open the tree view you will see which OIDs are available on that specific device. The standard for SNMP is an International Standards Organization (ISO) which specifies what values are stored in which OID. This is called a Management Information Block. There are a bunch of MIBs that are mostly extensions of the Internet Management MIB since there is a vendor specific area inside the ISO Internet Management MIB.
The dude comes with the Internet Management MIB and quite a few vendor specific MIBs. You can add others if you need to. The internet management MIB starts at 1.3.6.x. It is actually MIB-2 and as I said there is a vendor OID inside that which starts at 1.3.6.1.4.1.x. Cisco registered with the ISO and was given 9 so 1.3.6.1.4.1.9. is Cisco. Since the Dude automatically reads any OID under 1.3.6.x you do not need to add a MIB file that describes anything under that although if you do you will see the name of the OID instead of the number.
ISO Internet management mib-2
isosnmp.png
Vendor specific area, you can see I added the Cisco mib since you see the name instead of 1.3.6.1.4.1.9.x.x.x.x.x...
enterpries.png
There are other ISO MIBs (not Internet management-2 MIB) I would recommend never adding a MIB unless you know exactly why you are adding it. Also you almost never need to know the name of an OID you are wanting to use, just use the #.
So with out any changes to the dude every OID will start with 1.3.6. but the SSID will be under a vendor area. Cisco's SSID is stored at 1.3.6.1.4.1.9.9.272.1.1.1.6.1.2.1.X and since you can have multiple SSIDs x increments with each SSID.
snmpoid.png
Also you can see all the MIBs in the Mib Nodes panel.
wiki.mikrotik.com/wiki/Manual:The_Dude/MIB_Nodes
HTH,
Lebowski
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