Here is a simple why of how to import vendor SNMP MIB info to current Network Management System(NMS). In this case, PRTG.
First, download free SNMP importer from here.
1. Download SNMP MIB files(*.MIB) from vendor site or device itself
2. Open "ManagerPaessler MIB Importer" and import a downloaded file(NAS.mib in this example)
3. Once it clicked "Okay", below window will be popped up.
4. Verifying all MIB info you need.
5. Save as "NAS", it will become "NAS.oidlib"
6. Verify saved file on your folder.
7. Copy this file to directory /Program Files/PRTG Network Monitor/snmplibs on the server that PRTG is running on. (Important)