I think "DO" is "Digital Output". The fact the switch has Vcc and Gnd makes me think it's going to pull the output one way or the other if the switch is inactive, and then it goes the other way when it's active.
That's to save you from having to do the pulling up or down yourself, which of course on an Arduino is dead simple for the pullup case.
So, if you put 5V and ground on, what reading do you get on the DO pin (measured to ground) with the switch in its normal position and activated by the magnet?