With Setup Bluetooth Device not open, press and hold the power button on the aluminum wireless keyboard, paying attention to the fact that the green light does come on (otherwise you’re basically holding it in the off position!).
Select Setup Bluetooth Device from the Bluetooth menu icon. The keyboard is certain to be listed.
While still holding that button down, click Continue.
Don’t let go when you see the Connecting message!
Soon after, the set of numbers for you to type should appear. Now you can let go of the power, and the sync up should work correctly.
This is what it took to get an aluminum Apple wireless keyboard to pair with the desktop iMac – after I had paired that keyboard with an iPad and the keyboard bluetooth pairing wouldn’t let go of that sweet memory or something…