How to bias a tube amp with a Multimeter

On your multimeter, make sure the black lead is in the COM jack and the red one is in the proper jack for measuring voltage.  On some meters there are different jacks for the red to be inserted in depending on the function you select.  If your meter has only one input for the red probe, then that would not create problems.  If there are two input jacks for the red probe, be sure to select the correct one.

I am not familiar with your amp.  I take it there is no button or switch that needs to be turned on to do the biasing.  It appears that the common or ground probe goes into the center test point and the positive probe goes to the test point on either side of the center position.  It is a bit strange that the two positive test points have slots in them as if a screwdriver is suppose to go into that point.  Could it be that the point has to be screwed in before commencing testing?  

To be clear, I recommended trying to screw the test points, which have indentations for a blade-type screw driver; I was NOT talking about the adjustment pots.

Most bias adjustments are measured in milliamps, not millivolts. When you adjust the bias pot, you are changing the bias voltage, but, typically, you measure the current flowing which is milliamps.  Could it be that you are trying to measure the wrong thing?

Also, if you have a tech doing the adjustment, you might want to know how he determined the correct range for adjustment.  This depends on the design of the amp, so there is no single value that works well for all amps.

I thought that turning the screw would create a contact that would activate the probe point.  Your amp otherwise does not appear to have a switch to activate bias testing.  I could not think of any other reason why the test point has a groove that makes it appear to be a flat head screws.  

The picture you have of your VOM has, for the selector knob, 40A and 400A, which I assume is not 40 or 400 amps (crazy high current), but is 40 and 400 milliamps.  Select 400A and try testing, if the needle barely moves, then select 40A.  

It is easy to check the meter probe--just touch the two together when testing conductivity or resistance.  Probes are often pretty cheaply built and the wires, because they are constantly being moved and stretched, often do break.  That is a good observation of possible unexpected problems.