In the Interior Design for iPad app, to add objects like doors, windows, models, materials to your plan, all you have to do is drag and drop.

Just select the desired category on the Shelf (for example,  Living Room --> Seating), touch and drag any icon to the right, and that model will be added to your plan.

While in the Interior screen, you can add models and materials to your floor as well. The Interior Screen is also the only screen where you can add doors and windows.

To add a door/window, you'll need to move it to the wall. If the door is placed on the wall, it will be added. If the wall is too small for the door, then add the door to another wall and reduce its size.

You can watch this video tutorial: Adding windows 

You can also add models and materials on the 3D screens and assign materials to any model: How can I change materials of the objects?

Here's a FAQ on adding materials to any wall: How can I quickly change material of all walls in a room?