The MarbleWidget is also offered as a designer plugin. If you compile Marble yourself you probably need to enable the compilation of the designer plugin:
If your source directory is "~/marble" and your build directory is "~/marble_build" then you can enable compilation of the designer plugin via the cmake call:
cmake -DWITH_DESIGNER_PLUGIN=ON ../marble
Once you have compiled marble using make you can install everything using "make install". This should install the file
into Qt's designer plugin directory (please check this in case things don't work out).
Once you start Qt Designer you should spot a new "Marble Desktop Globe" category in the widget box on the left. You can just use this MarbleWidget like all other widgets by dragging it onto your form and by adjusting the properties of the MarbleWidget.
As a ready-made example you can load the two examples in:
The examples nicely demonstrate how signals and slots work with the MarbleWidget.