The ohmmeter in the common meters will not check the diode either. The voltage drop across the diode in the microwave is more than the meter is set up to measure in the ohms range. It is not around .7 volts as one would find in the lower voltage diodes. Internally the diode is made up of several diodes so the voltage drop is much more.

correct

if the diode is good, with your symptoms i would next check the continuity of the filament in the magnetron or look for a blown fuse someplace

DO THIS WITH THE POWER DISCONNECTED
Mark