The cursor may have a small symbol of the element that you want to insert. For example if you select a window, the cursor will have a +with a small picture of a window to show that it is ready to be inserted.
The cursor may turn into an arrow with a X, which means you are at an intersection. For example, connecting two walls.
If the cursor is an arrow with an X inside of a circle this means you are inserting at a midpoint.
If it is just a + you can insert by picking points.