Apple iMac (21.5-inch and 27-inch, Mid 2010) - Manual (Page 48)

48 Chapter 4 Problem, Meet Solution
3 Slide two AA batteries into the battery compartment, as shown below.
Insert batteries
4 Replace the battery compartment cover.
To check the battery status, press the power (®) button. If the indicator light doesn’t
glow, you may need to change the batteries. You can check the battery level in
Keyboard preferences. Choose Apple () > System Preferences and click Keyboard.
The battery level is in the lower-left corner.
To replace the batteries in the Apple Magic Mouse:
1 Slide the on/off switch on the bottom of your mouse to turn it off.
2 Slide the latch down and remove the battery cover.
Latch Batteries