Chances are the portals is "outside" the map in a position where you can't get to it. With other maps, it was their error that they didn't move it or it's there you just have to look harder. For your own maps, on the first tab where it says: Has Portal? If that is clicked, there are X Y and Z coordinates below. Move your cursor around to where you want it and at the bottom right it shows coordinates of where your cursor is. Copy the first three numbers on a piece of paper. Then put the first number in the X slot, second in the Y slot and third in the Z slot. For the arrival yaw, you can put any number, it's not as important.