Your MK550 Desktop combo ships with an M510 Wireless Mouse. The mouse is pre-set to be on and working, once you remove the little packing strip that preserves the initial battery charge.
To turn your mouse off:
Flip the mouse over and locate the power switch.
Slide the switch to the OFF position as shown below. You'll see a red background to the right of the switch..
To turn your mouse back on::
Flip the mouse over and locate the power switch..
Slide the switch to the ON position as shown below. You'll see a green background to the left of the switch to indicate that the mouse is on.
If you flip the mouse back over quickly, you'll see that the green battery indicator light is lit. It will only stay on for a few seconds though before going off again to conserve battery power.