All Masso units are tested before being sent out so it is a bit strange that it isn't working.
Connecting a wire from Estop pin 2 to the +ve of the power supply should show the Estop input go high and removing it should make it go low. The input should not be inverted.
Is the Estop indication on the screen flashing Yellow or Red? It is normal for the Estop to require resetting when you start Masso so simply simply connecting a wire between +ve and Estop will not remove the Estop on startup. You ultimately must install a switch so you can reset it.
Could you just review this video and see if it helps with the issue. Let us know your findings and if you are still having a problem we can investigate further.