Hard to tell what's going on from the screenshot. But you need to paint a level with the segment of your choice. Then select from the toolbar whatever icon makes you paint floors only. Then right-click where you want to put a hole in the floor to go down a level. Then hit the '-' to move down a level. Select a segment, paint the level, and select stairs, then put them under the hole you put on the upper level. That will allow you to go up and down stairs.
This is basic stuff in the manual. Read it thoroughly. Be sure to do the tutorials, and read Nicky's guide which should be a sticky at the top of this forum.
"I'm trying to find new ways to make this game more attractive." - Dennis Rodman