Used boot camp for windows - cannot select partition at boot up
I believe once you install Boot Camp, the computer will boot into Windows by default unless you change it. Presumably, you're holding down the option key immediately on startup, it should be pressed as you hear the startup chime? 

Apart from holding down the option key at startup, if you find you can only boot into Windows, this Apple support page tells you how you can change the default startup volume from Control Panel.


  This startup volume selection is temporary.  To set the default startup volume for your Mac after your computer is running, open System Preferences and choose Startup Disk from the View menu (in Mac OS X) or open the Boot Camp control panel (in Windows).


Also, do any of the other Startup key combinations work, e.g. Safe Boot or Single User Mode?