Edit: Better Link
http://support.apple.com/kb/TS3273
Resolution
Apple Wireless Keyboards will not recognize pressing and holding down startup keys during startup until the startup chime sound is heard. This is normal behavior. You may find it helpful to press and hold startup key combinations while the startup chime is playing. Two common startup key combinations are listed below.
Maybe you are not waiting to press the keys until the startup sound?
https://discussions.apple.com/thread/3716140?start=0&tstart=0
Feb 6, 2012 1:14 PM
I have an Intel iMac that came with a bluetooth keyboard and mouse. There are a variety of options that can be used when you boot the Mac, such as hold down the C key to boot from a CD/DVD, hold down the option key to choose the boot drive, etc. When you have a BT keyboard, it doesn't connect until it is booted. How do you get the iMac to recognize, say, the Option key?
iMac, Mac OS X (10.7.2)
This solved my question by captfred on Feb 6, 2012 2:18 PM
You wait for the startup tone, then quickly press and hold the key until the logo with pinwheel. Pressing too early will ignore the input.
Regards,
Captfred