OS X Yosemite: Pair an Apple wireless keyboard, mouse, or trackpad with your Mac
Use Bluetooth preferences to pair your Mac with an Apple wireless keyboard, mouse, or trackpad.
Make sure the batteries are properly inserted in the keyboard, mouse, or trackpad and the device is turned on and discoverable. See the documentation that came with the device for more information.
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Choose Apple menu > System Preferences, then click Bluetooth.
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Select the keyboard, mouse, or trackpad in the Devices list.
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Follow the onscreen instructions to set up your keyboard, mouse, or trackpad.
If you replace the batteries in a keyboard, mouse, or trackpad that is paired with your Mac, click the mouse or trackpad or press a key on the keyboard to reconnect it to your Mac.
For more help using Bluetooth, click the Help button 
in Bluetooth preferences.