

Copy the header and source files into your project directory. From there, add them to your .pro file. Go to QtCreator, drag and drop a "Widget" widget onto your UI and scale it to the size of the custom widget you want to use. Right click it and click "Promote To". Then type in the name of the header file, click "Add" and then select it from the list. Then click "promote."
For a video tutorial, see here: http://youtu.be/VyVKbZAriK4
(I know how old these comments are, so I'm leaving this here for other people who may come across this issue.) - Feb 10 2012
For a video tutorial, see here: http://youtu.be/VyVKbZAriK4
(I know how old these comments are, so I'm leaving this here for other people who may come across this issue.) - Feb 10 2012