On the back of your tachometer there are three connections, plus sometimes an extra wire for the dashboard/instrument lights if it has one.

I = Purple wire (ignition)
S = Grey wire (signal pulses from stator)
Gnd = Black wire (ground to battery)

Optional, is a Blue wire that might be connected to the +voltage side of the little light bulb.

If your wires are totally missing, you might peek at the front of your shift control box and see if there is a three pin connector there. You may get lucky and just need to buy a tachometer harness from an Evinrude/Johnson dealer and plug it in there.

Otherwise, you can run wires all the way from the motor. There is a screw terminal strip on the side of the engine block where you can conveniently pick up all three colored wires right there and run them to your tach.