Pairing wireless iMac keyboard to iPad 4

First, you need to unpair it from your iMac:

1. Press and release the On/Off switch on the wireless keyboard to turn it off.
2. On the currently-paired Mac, choose System Preferences from the Apple () menu, then choose Bluetoothfrom the View menu.
3. Select the Apple Wireless Keyboard on the left side of the Bluetooth preferences pane.
4. Click Delete ➖ in the lower-left corner.

Then you need to make the keyboard discoverable:

1. Press and release the On/Off switch to turn on your wireless keyboard.
2. The green LED should begin blinking if the keyboard is not already paired and connected to another computer. The blinking LED indicates that the keyboard is in the discoverable mode. If you don't pair your keyboard with your Mac within three minutes, the indicator light and keyboard will turn off to preserve battery life. If this happens, press the On/Off switch to turn your keyboard on again. Note: If the LED doesn't turn on, make sure you have good batteries in the keyboard and that they are properly installed.

And then follow these steps on your iPad:

1. From the Home screen, choose Settings > Bluetooth2 and turn Bluetooth on. iPad searches for nearby Bluetooth accessories.
2. Choose the Bluetooth accessory, and then enter a passkey or PIN if prompted.
   • Bluetooth headset: Refer to the documentation for the headset to determine the passkey.
   • Bluetooth keyboard: Type the passcode displayed on the iPad on the keyboard and then return.
3. When pairing is complete, you can use the Bluetooth accessory with your iPad.

From http://support.apple.com/kb/HT4111 and http://support.apple.com/kb/HT1809

I ise mine with my ipad2 make sure its 100% disconnected from the mac a keyboard cant be connected to more then one device