Okay I found the code, put it in, but nothing happened - once you start a game that "enter the code" dialogue on the main screen doesn't appear again.
Yes if you’re already advanced in the game, the dialogue won’t show up anymore, it’s really to obtain a quickstart so you would need to erase your whole progression for that and the dialogue will re-appear, but it only gives you the 5 first meta-insights points in the end, so if you’re already passed that stage in your game, there is no point in using the code now…