How to select an operating system to boot without a keyboard?
If you have an Apple Remote for your Mac, you can try the following:


shut down your Mac (by holding the Power button for a few seconds, if necessary)
press and hold the Menu button on the Apple Remote
start your Mac by pressing the Power button
release the Menu button on the Apple Remote


You should now be able to choose a startup volume.

If your keyboard has still not connected to your Mac via Bluetooth, you can select the startup volume by pressing the Left and Right button on the remote. To confirm and boot from the highlighted volume press the Select button.