We installed the latest version of the plugin on Zen Cart 1.5.4.
After the initial difficulties to configure a test sandbox, we can now checkout the order, reach myPOS, make the payment.
A major issue remains:

for whatever reason, myPOS cannot issue the correct callback to let Zen Cart know that the payment was accepted, thus making it impossible to change the order status, notify the user and empty the cart.

Since the plugin shows to have been downloaded 528 times, I was wondering how everybody else has managed to address this issue since, from my point of view, it makes the plugin unusable.