You should not need MIBs for PolyMon to work - you only really need the OID, plus you need to make sure you specify the correct public (read) community (basically password - but that's probably simplifying the explanation - SNMP is not my thing either).
Finding the OID's can be a challenge but that's where MIB libraries and a 3rd party MIB browser can come in handy. Either that or a Google search - but that's sometimes quite hard too.
For example, for our APC environmental monitoring unit (EMU), the 1st temperature sensor's OID is: 220.127.116.11.4.1.318.104.22.168.1.1.0
In PolyMon you would have to specify the host IP (or name), port (in our case the EMU's port is 161) and the password to access the data. That's all that theoretically is needed.