Jim is correct. Info is in the Driver's Guide app, which has replaced the paper manual for me. The buttons are identified in:
Quick Reference Guide > Displays and control elements > Steering wheel buttons, left
The functionality is explained under Camera-based Cruise Control with Stop & Go, ACC. From the start screen tap the magnifying glass icon, you'll see a list of topics. ACC is second on that list.
Note: The i3 steering wheel buttons vary depending upon the equipment fitted to your car and/or global region. I'm guessing you're in North America since we get ACC but not traffic jam assistant, which requires different buttons. However it might help people answer future questions if you add a location to your profile.