Error: top level design entity " " is undefined

I encountered exactly the same problem. I verified Assignment->Settings (with my project name)->General/Files/Device and everything looked fine. I also did Project->Add/Remove Files in Project... and everything looked normal. I had a very simple schematic file under the same directory with the project file (directory and project file have the same name). I wanted to try out with the simple one first since do not want to spend time on the complete project before I know it would work. I used the New Project Wizard and went over their tutorial several times. Can anybody explain to me more since I am at dead end. I am new to Altera and this is the free version with Cyclone II. I also tried to call Altera Tech Support but got the recorder taking me around the loop. Thank a bunch.