A friend of mine took it recently and I was told it's the same entrance as the other Business/First class bus entrance (where you'd normally board the train when it was in service). You just have to let them know you want the BMW i7 service instead of the bus (only if you are arriving on MH J). It's not by C1 as mentioned by MH (they could really hire a new social media manager).

And MH J i7 pax will now be dropped off at the same area too (below the train stop) instead of near gate C11. I am curious if there's a lift that can bring you right up to the lounge since the drop-off area is right below the lounge.