When you use your Apple Wireless Keyboard and Mouse to start up your computer to the Startup Manager, you should still be able to use the Apple Wireless Mouse to select a startup volume.
If you find that your Apple Wireless Mouse isn't working in this situation, you can use the arrow keys on your keyboard to select your preferred startup volume. When you have selected the volume you want, press the Return key.
If you are starting up to the Install disc that came with your Bluetooth-equipped Macintosh computer, the pairing assistant will appear and ask you to put your mouse into discovery mode so it can be paired and used when started up from the Install disc.