Bootcamp and wireless usb keyboard: How to boot with Mac os
Boot into windows and use the Bootcanp control panel to set the next boot to be Mac OS X. 

From http://support.apple.com/kb/HT5639 :


  Startup questions
  How do I choose which operating system will start up my Mac?
  After running Boot Camp Assistant and installing
  Microsoft Windows and Boot Camp drivers, press and hold the Option
  (Alt) key while you start up your Mac to switch between Microsoft
  Windows and OS X. 
  
  Use the Startup Disk Control Panel in Windows or OS X Startup Disk
  System Preferences to set the default operating system to use each
  time the computer starts up.


Or you can get any old USB keyboard and hold the option key.