The error you posted now is a link error.
elease/MainWindow.o:MainWindow.cpp.text$_ZN13Ui_MainWindow7setupUiEP11QMainWindow[Ui_MainWindow::setupUi(QMainWindow*)]+0x129): undefined reference to `MyCustomWidget::MyCustomWidget(QWidget*)'
But error you posted at the beginngin of this thread was a compile error:
debug/MainWindow.o:C:\QTProjects\cccc-build-desktop/./ui_MainWindow.h:43: undefined reference to `MyCustomWidget::MyCustomWidget(QWidget*)'
You have to be a bit more cooperative if you want effective help.

Your link line is not good:
-L"c:\Qt\4.7.2\lib" -lmingw32 -lqtmain C:/QTProjects/MyCustomWidget-build-desktop/release/mycustomwidgetplugin.dll -lQtGui4 -lQtCore4
'-l' is missing before 'C:/QTProjects/MyCustomWidget-build-desktop/release/mycustomwidgetplugin.dll'