The -r- reading in the Multimeter indicates an overrange. That means that the actual reading is greater than the Multimeter's setting. For example, if you are measuring a 1.5 GOhms resistance but the Ohmmeter overrange resistance is 1 GOhm, the display cannot show the number and the -r- is shown. The same happens for voltage or current readings.
To change the display settings just double-click the Multimeter and click on Set. Once you change the Ohmmeter overrange resistance to large enough value, you should be able to see the diode's resistance.
Regards,
Camilo V.
National Instruments