Connect an Apple wireless accessory with Mac
To use your Apple wireless keyboard, mouse, or trackpad, you first need to connect the device with your Mac to enable Bluetooth.
Connect a Magic Keyboard, Magic Mouse 2, or Magic Trackpad 2 with your Mac
When you connect your keyboard, mouse, or trackpad to your Mac, it automatically connects with the Mac using Bluetooth.
Connect one end of a Lightning to USB Cable to your Magic Keyboard, Magic Mouse 2, or Magic Trackpad 2 and the other end to the USB port on your Mac. Depending on your Mac, you may need an adapter, which is available from an Apple Store or on apple.com.
You can use your Magic Keyboard or Magic Trackpad 2 while it’s connected to your Mac. You can’t use your Magic Mouse 2 while it’s connected.
When you use Bluetooth to connect your keyboard, mouse, or trackpad with your Mac, you receive a notification that you can disconnect the cable and use your device wirelessly.
Connect an Apple Wireless Keyboard, Magic Mouse, or Magic Trackpad with your Mac
Make sure the batteries are charged and properly inserted in the keyboard, mouse, or trackpad and the device is turned on and discoverable.
On your Mac, choose Apple menu
Select the keyboard, mouse, or trackpad in the Devices list, then click Connect.
If you replace the batteries in a keyboard, mouse, or trackpad, click the mouse or trackpad or press a key on the keyboard to reconnect it with your Mac.
For more help using Bluetooth, click the Help button 
in Bluetooth preferences.