Best bet if this is unclear is simply to contact Grin.
I've never installed one of these, but here's a manual I have on the installation that seems pretty clear. You have two things to align:
- the sensor in the BB shell so chain tension pulls against the sensor in the correct direction and
- the crank on the taper so the innards apply force to the sensor correctly.
So there are two things to line up:
- the pin on the housing and
- the indentation of the shaft taper.
According to this manual, both the pin and indent need to be aligned forward and 180 degrees away from a line from BB to rear axle.
Here's some shots from the doc as well as the doc itself.
The last image clearly shows the red forward mark on the BB shell and the forward-facing indent on the taper.