Hello,
Go to System Preferences/Bluetooth and follow these instructions to re-pair the MM:
You can set up an Apple wireless keyboard and mouse using Bluetooth Setup Assistant.
Make sure the batteries are properly inserted in the keyboard and mouse, and the keyboard and mouse are turned on and discoverable. See the documentation that came with the keyboard and mouse for more information.
To use Bluetooth Setup Assistant:
Choose Apple menu > System Preferences, and then click Bluetooth.
Click Devices, and then click Set Up New Device.
Follow the onscreen instructions to set up your keyboard or mouse.
If you have replaced the batteries in a mouse that is paired with your computer, click the mouse to reconnect to it.
For more help using Bluetooth, click the Help button in the bottom-right corner of Bluetooth preferences.
"The next time I had to turn it iff to change the batteries it would not automatically pair when I turned it back on" I've had good luck when changing out the batteries to move the slider on the back up and down one time, that usually will re-pair the mouse for you, if not, click the Bluetooth icon in your Status Menu Bar, click Browse Device. Select the Mighty Mouse in the list and click Browse. That will reconnect the the mouse for you.
Carolyn 🙂