The tube wires like this. Positive on the mirror end and negative on the output end.
A current meter is strongly recommended to prevent over powering the tube. Tubes have a maximum current and most power supplies can deliver more current than tube can handle. Excessive current will adversely affect the life of the tube. When setting up the tube you will slowly turn up the power until you reach the maximum then record that level and try not to exceed it in normal use.
The meter must be connected on the negative side or the whole meter will float at high voltage be extremely dangerous.
There are two types of meters, analog and digital. Either one is sufficient for setting the maximum power. Digital meters have a nice modern look, but are more complicated to hook up. I prefer analog because they give better feedback when the laser is engraving or rapidly changing power.