My OS is Win7 and I've installed Qt 5.5.0
I create a simple custom widget plugin New Project->Other Project -> Qt Custom Designer Widget
after creating the dll , I copy it in D:\Qt\Qt5.5.0\Tools\QtCreator\bin\plugins\designer and D:\Qt\Qt5.5.0\5.5\mingw492_32\plugins\designer
but in Qt Creator designer does not appear my widget in the widget box
but if I run designer.exe, all ok , my widget is displayed
i read Qt Designer doesn't load my custom widget plugin and i read How to promote a custom widget in QT Creator
I came to the conclusion that qt creator does not work correctly
Sorry I do not have enough reputation to comment. There are many possible reasons, why your plugin does not load. You can look for further information by opening a *.ui file in qt creator and then take a look in the menu: Tools->Form Editor->About QtDesigner Plugins.
If your plugin is listed there, you copied it in the right folder and you will get a hint why it does not load. In my experience, the most common mistake is, that your qt-creator was not build with mingw, but you are trying to load a plugin built with mingw. You need to use the same compiler for both.