Its been a little while since I did one of these, but here goes....
There's a set of 3 plastic bars that probably read in the instructions as separate pieces but are likely already screwed into the top or bottom of the extra floor. It doesnt fit because the building already has a set installed from where you removed the ground floor.
Do you have those?
*If* I remember right, you need to remove them from the addition, screw them onto the building after you've split the ground floor from the rest of the building then screw in the additional floor to the bars you just added.
As for the light, it seems like you should have a socket to plug in the lighting connection that got disconnected before you added the floor. That may require a call to MTH.