AP is correct. It's not an option on the M5 or M6. Firstly, the steering wheel doesn't have the button space to support the distance toggle. On my old F10 550 with ACC the distance toggle buttons were where M1 and M2 are now. (Though I'm sure they could get around this somehow.)
Second, the sensor would obstruct airflow in the front of the vehicle. That's why we have no fog lights, for example. That isn't to say they couldn't find a way to make it work, but they have chosen not to - and not just on the M5. I noticed on the F30 you can't add ACC with the M Sport package, and you also (again) lose fog lights.
What I do find curious though is that our instrument clusters actually have the indicators in them for ACC. There are 5 blocks in between the diagonal lines used to indicate that lane departure warning is active. The car symbol used by the collision avoidance system is also shared with ACC. It might be a cost-saving thing or perhaps wishful thinking for future.