Your CDI looks like the older single channel CDI unit, p/n 41004. You can find the wiring sheet here
http://motec.com.au/filedownload.php/M33-1.pdf?docid=1024.
There are two options for driving the tacho, you can either use the ignition pulse to the CDI on pin 33 of the ECU, or you can configure a dedicated tacho output from the ECU if you have a spare output.
It is better to have a dedicated output for the tacho, especially if you are going to connect a monster tacho.
For the original tacho to work, you should just spice the wire that went to the standard ECU as the 'tacho signal', either to pin 33 on the M4, or another spare output configured as the tacho output. It should just work.
The monster tacho then also splices to this wire, to the 'tacho signal' input. Just make sure the monster tacho you get has an EFI ignition signal input, and not a coil negative input only.