Hi,
Do you have a DMM?
Do you have gnd of the arduino connected to Vin- of the other supply?
It doesn't look like it, so the MOSFET cannot turn ON due to no gnd reference to the arduino signal.
Tom..... 
Hi,
Do you have a DMM?
Do you have gnd of the arduino connected to Vin- of the other supply?
It doesn't look like it, so the MOSFET cannot turn ON due to no gnd reference to the arduino signal.
Tom.....