When you turn on an unpaired Apple wireless keyboard, it automatically goes into discoverable mode for three minutes, flashing its green LED to show its discoverable status. After three minutes of inactivity, it exits discoverable mode and stops flashing the LED to save battery power, so you must press the power button again to put it back into discoverable mode.
Enter Discoverable Mode After Pairing
An Apple keyboard supports pairing with only one device at a time, and pressing its Power button while its paired device is on and within 33 feet turns the keyboard off rather than enabling discoverable mode. Additionally, it doesn't enter discoverable mode once it detects its paired device after being turned on. Before pairing it with a different device, turn it off and delete its Bluetooth connection from the currently paired device. When you turn it on again, it enters discoverable mode because it doesn't detect a paired device.
It's possible to unpair the keyboard simply by turning off its paired device, but deleting the Bluetooth connection prevents conflicts and erratic behavior when two paired devices are turned on. Even if paired with several devices already, the keyboard enters discoverable mode when turned on if none of the existing devices are on. However, it doesn't need to be discoverable to connect to an already paired device, and it automatically connects to the first paired device turned on within range.
Pair With Any Device
Apple wireless keyboards work with most new devices, including Windows and Mac computers and iOS and Android devices. Refer to the Apple website for Mac software requirements for each wireless keyboard model (link in Resources). With your keyboard in discoverable mode, open your device's Bluetooth setup utility. On a Mac, open System Preferences, choose "Keyboard" and click "Set Up Bluetooth Keyboard." In iOS or Android, turn on "Bluetooth" in Settings, and in Windows, open Control Panel and select "Add a Device." Your device shows a pass code and a countdown timer, and you must type the numeric code on the keyboard and, to pair the two devices, press "Enter" before time runs out. Once paired, your computer or mobile device remembers the connection, and you don't need to pair them again.