Apple iMac (21.5 and 27-inch, Late 2009) - Manual (Page 48)

48 Chapter 4 Problem, Meet Solution
3 Slide two AA batteries into the battery compartment, as shown below.
4 Replace the battery compartment cover.
To check the battery status, press the On/off (®) button. If the indicator light doesn’t
illuminate, 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.
Insert batteries
Latch Batteries