Circuit with switching MOSFET not working
For starters, you seem to be shorting the "Arduino Digital Output" to ground whenever you close the switch (push the pushbutton).  

Secondly, the n-mos is not connected properly, it should be placed with its source to ground and its drain to the load.  The ammeter should also be on the high side, not the low side, because it interferes with the Vgs being applied. 

Thirdly (some may put this as first in priority), when you put a voltage source in your schematic, don't place it upside down because it obfuscates the schematic (I wish the "rotate" tool in schematic entry programs was disabled in such cases).