In the following video you will find instructions for installing the 7th channel feature on the Vision+ (find it in Pdf here)

Installation of 7th channel lever on the DJI remote controller
> lever installation

Note:
> We advise to turn on the remote before turning on the Phantom (and also don’t touch the slider during startup sequence).
> Always connect Vision App when using 7th channel. When Vision App is not connected the 7th channel will not work properly.