When using sensor "SNMP Custom" (capable of querying one OID), you can set this directly on the sensor's "Settings" tab. With sensor "SNMP Custom Advanced" (capable of querying up to 10 OIDs) you need to set unit to "Custom" on sensor's "Settings" tab for the regarding channel. After that let the sensor run one scan, then click on the regarding channels's gauge on "Overview" tab, then you can apply multiplication/division in the channel's settings.
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.