It needs to be set to read mV, that's millivolts. To be specific, it will be reading a negative mV rating. I believe that your amp has a 3 pin connector and two trim pots for setting the bias in the back of the amp. The black lead from the multimeter connects to the center pin. You then connect the red lead to the pin on whichever side you're adjusting. You do one side at a time. Don't take a reading off the amp until it has warmed up and is set ready to play through, ie, standby switch is not engaged. Be sure you have your speaker cabinet plugged in as well.
I think the bias on 100V marshalls, the DSL and TSL is supposed to be -90mV per side. Double check that with a tech first though, it's been a few years since I did it.