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.