Your coil's negative terminal can be used to send the engine speed signal (aka. "the tach signal") to your MicroSquirt® controller. However, you can ONLY do this if you are using your MicroSquirt® controller to adjust the fuel only. If you want to control ignition timing advance as well, you cannot use the coil's negative terminal, and must use a distributor, crank wheel, or cam wheel.
If you are triggering directly off the coil (instead of something like a missing tooth wheel or another relatively low-voltage signal), connect OPTOIN+ (Ampseal pin #30) to the coil's negative terminal, and connect the OPTOIN- (Ampseal pin #31) to a 12V source (rather than grounding it as some older instructions recommend).
This then uses the flyback spike off the coil as a trigger (rather than the 12 Volt signal). Doing this reduces the energy dissipated in the circuit, prevent a thermal 'melt-down' of the components inside your MicroSquirt® controller.
Note that you CANNOT connect any of your MicroSquirt® controller's pins directly to a CDI (capacitive discharge ignition) coil's negative terminal in any fashion - doing so will damage your MicroSquirt® controller. CDI ignitions include MSD 6a, some motorcycle ignitions, etc. Often such ignitions can be made to work with appropriate circuits, check the documentation.