Apple iMac (21.5 and 27-inch, Late 2009) - Manual (Page 49)

Chapter 4 Problem, Meet Solution 49
3 Insert the batteries with the positive ends facing up, as shown above.
4 Replace the cover and turn on your mouse.
You may have to click your Apple Magic Mouse once to reconnect it to your iMac.
To check the battery status, slide the On/off (®) switch to turn on the mouse. If the
indicator light doesn’t glow, you may need to change the batteries. You can check the
battery level in Mouse Preferences. Choose Apple () > System Preferences and click
Mouse. The battery level is in the lower-left corner.
Using Apple Hardware Test
If you suspect a problem with your iMac hardware, use the Apple Hardware Test
application to help determine if there’s a problem with one of the computer’s
components, such as the memory or processor.
To use Apple Hardware Test:
1 Disconnect all external devices from your iMac, except a USB keyboard and mouse,
if you use them. If you have an Ethernet cable connected, disconnect it.
2 Restart your iMac while holding down the D key.
3 When the Apple Hardware Test language chooser screen appears, select the language
you want to use.
4 Press the Return key or click the right arrow button.
5 When the Apple Hardware Test main screen appears (after about 45 seconds), follow
the onscreen instructions.