Hi,
I had the same problem as a lot of the people submitting to this forum (where my bluetooth keyboard would NOT turn on...no green LED light, and no recognition from my mac)....I thought I might have damaged it by putting in replacement batteries incorrectly, and was about to throw it out, but then!!!

....I launched the Keyboard Preferences from the bluetooth menu, where my keyboard was NOT connected....then I selected "Set Up Bluetooth Keyboard" button, and voila, the green light appeared briefly....and my keyboard sprang to life!
I obviously don't want to put myself through this again, but if these instructions help anyone, then at least there will be one less keyboard being thrown away, needlessly.
Good luck to everyone, James