using Microsoft Arc Mouse surface edition on MacBook Air 2015
Found a solution.


Open system settings/bluetooth.
switch bluetooth off and back on again, this way it will start scanning for new bluetooth devices
click and hold (for 5 seconds) the button on the mouse to start the pairing process. The led lights on the mouse will flash red and green
After a few seconds the mouse appears on screen, and next to it a big "PAIR" button. 
Click it! :)