Apple iMac (21.5-inch and 27-inch, Mid 2010) - Manual (Page 13)

13 Chapter 1 Ready, Set Up, Go
Step 4: Press the power (®) button on the back to turn on your iMac.
You hear a tone when you turn on your iMac. Click your mouse to establish a
connection with your iMac.
®
®
®
Important: If your keyboard and mouse aren’t paired, follow the onscreen instructions
to pair them with your iMac. You may have to click the mouse once to establish a
connection. For more information about pairing, see page 27.