A UI-less fork of SensibleSideButtons for Mac OS X 10.8+
Instructions
1. Mount the DMG, and copy Side Button Enabler to your Utilities folder.
(Or to any other location on your hard drive.)
2. Grant Side Button Enabler permission to control your computer:
Mac OS X 10.8::
Open System Preferences → Accessability, and check the box labeled "Enable access for assistive devices"
Mac OS X 10.9 and later::
Open System Preferences → Security & Privacy → Accessability, and add Side Button Enabler to the list of apps allowed to control your computer.
3. Tell you Mac to open Side Button Enabler at log in:
Open System Preferences → Users & Groups → Login Items, and add Side Bar Enabler to the list of apps that open when you log in.
4. Run the Side Button Enabler app.
The M4 and M5 buttons on your mouse should now work as Forward and Back buttons. Hold down the middle mouse button (M3) to enable side buttons's native functionality (useful if the side buttons are set up as shortcuts in Mission Control).
Troubleshooting
Buttons don't work? Try using Activity Monitor to quit Side Button Enabler, and then open it again. If they still don't work, double check that Side Button Enabler is allowed to control your computer.
Theoretical FAQs
Q: Why did you make this?
A: Primarily because I wanted to use SensibleSideButtons on Mavericks. The original version is only compatible with 10.10 Yosemite and above. I also added the ability to hold down the middle mouse button to enable the side buttons's native functionaity, which allows me to use the Mission Control shortcuts I set up in System Preferences.
Q: Why is there no UI?
A: The original app's UI was built using storyboards. Storyboards don't work on Mavericks.
Q: This app is great, who do I thank?
A: The original developer, archagon. Go to http://sensible-side-buttons.archagon.net for more information. He accepts donations.
Q: This app is awful, who do I hate?
A: Definately not archagon, please don't contact him, we have no relation! You can hate me, if you want, as long as you don't tell me. It would hurt my feelings. :(