If the paper from Symtech states the IAC2 - blue/white wire is for TACHO output then it is to be used for your stock tachometer or an aftermarket tach if you have one.

The High Current GPO IAC2 - white/green is for whatever you want to drive or switch on/off by the MS.

Are you going to run ignition with your MS? If so then you need to connect your ignition output wires to you ignition coil in order to drive it.

If you appear to be missing a wire, I would contact Symtech and speak with them about your harness. Ideally you should have checked for all the needed wires prior to installing/soldering your harness. If you need to send the harness back in for service, you need to cut it back out to remove it, if you are in fact missing a wire.