Hi Michael
I'm assuming you are using an MPG pendant purchased direct from Masso or are using one that is wired the same as per the circuit diagram.
https://www.masso.com.au/masso-documentation/?section=e-stop
I have to admit this took me a little bit of head scratching when I first set mine up.
If you are using only the Pendant then you need to connect the Masso positive of the power supply to EStop pin 1 and then your pendant EStop button will work. EStop pin2 is left disconnected when using a pendant.
If you decide to add an external EStop button at a later date just put it in series with the +ve of the power supply and EStop pin 1.
After powering up Masso the Estop will be flashing and you just need to cycle the EStop button to reset. This is done to test to make sure it is working before machining begins.
Hope this helps
Cheers
Peter