It's kinda simple: First of all you go to the very top left corner(Invite your party members) and use the searching item (Dunno how it is called) and after searching one time you go into that secret room(That's the only portal right below the very left top corner) and search there for a signal(IMPORTANT is that you don't find the "point" in that room instatnly, you have to get the Arrow above your head). After that you go back to the position where you searched for the first time and you should find the "point" very near to that position.