My first guess would be the version of your QtCreator. In the setup guide it is recommended to download QtCreator from their page directly instead of using the version shipped with your distribution (which is probably outdated).
The guide recommends version 3.5.1
http://openframeworks.cc/setup/qtcreator/
Which version are you running?
Best reinstall QtCreator and stick with the guide.
Best,
Felix
