The massager was made for 110 V use, you doubled the voltage and burned it out. You might be lucky if the unit is a low voltage unit and relies on a 12V power inverter and when you plugged it in it ruined just the inverter.
Are you moving your arm at all after you press the start button. I just have a wrist cuff, but if I move anything after I've pushed the start button I get an error code.
400ma is not a lot of juice. Does the BP monitor have a battery compartment? IF so, just add 1.5 volts for each battery, rounding up. The BPM should have a number itself, where the adapter plugs in, and a "+" with a line pointing to the middle or the outer ring indicating polarity.
In general
2 batteries= 3V-3.7V rated adapter sometimes 5V
4 = 3V, 6V,
