Hello, Kerim,
To use a non-vanilla Qt class in Qt Creator you can resort to something called “promotion”. You add, for example a QWidget, then select it in the widgets tree and choose “promote to” in the context menu. A dialog will appear, prompting you to enter the name of the class and to choose the header file that defines it. More details on the promotion process can be found here. This the easy way to do it, but the drawback is that Qt Creator will not be aware of any custom properties the new class may have.
To have full access to QVTKOpenGLWidget's properties, you need to add VTK’s Qt Creator plug-in .DLL/.SO to the plugins directory in Qt Creator installation directory. Details on this you can find here.
regards,
Paulo