The sources of the Freebox-Qt plugin are on github. You can build the plugin yourself, or download the precompiled shared libraries below.
These precompiled shared libraries are built for QtCreator 4.1.
For Linux 64 bit. Place it in $QT_DIR/Tools/QtCreator/lib/qtcreator/plugins/libFreebox.so
For Windows 7+ 32/64 bit. Place it in $QT_DIR\Tools\QtCreator\lib\qtcreator\plugins\Freebox4.dll
For Mac OS X, coming soon.
Finally, you must download this QML library. It is architecture independant (no executable). Choose either the 7zip file or the tar-compressed file and place its content (the fbx directory) in $QT_DIR/5.7/gcc_64/qml/ (for Linux) or $QT_DIR\5.7\msvc2013\qml\ (for Windows).
The documentation of this library is here.