Qt Creator IDE - Qt Designer for Windows

In case you need to use Qt Designer for Windows and you just use it occasionaly or cannot invest too much, you can use Open Source version. This can be downloaded from here (choose OpenSource):
https://info.qt.io/download-qt-for-application-development
or here:
http://download.cnet.com/Qt-Creator-IDE/3000-2212_4-75300653.html
Then if you are just starting to learn this tool and Design tab is greyed out, make sure that you select the *.ui file from the files dropdown in Edit mode, usualy its mainwindow.ui. Then the Design mode should become active and you can start easy visual creation of the GUI.