NI-DMM 2.5 was released with LabVIEW 8.0 and does not include support LabVIEW 8.20 because it was released later. Therefore a new driver version, NI-DMM 2.5.1, was released with LabVIEW 8.20 so that they can be used together. This driver can be found on the current Driver CD. If you received this CD with LabVIEW just install the 2.5.1 driver from it. There is not a separate installation build for this driver so that is why it does not currently appear on the Drivers and Updates page. If you don't have the Driver CD please contact National Instruments.
Have a great day,
Brian P.