The question you linked explains this very well. There is no such thing as PyQt Designer. There is only Qt Designer, which must be installed with Qt.
PyQt provides a plugin that allows custom widgets to be written in python for Qt Designer. This plugin is installed by default when PyQt is installed, unless it has been explicitly configured using the --no-designer-plugin option.
If you run Qt Designer and go to Help> About Plugins, you will see some python or pyqt plugin listed there on Mac OSX, I think the plugin should be installed in /Developer/Applications/Qt/plugins/designer/ ).
ekhumoro
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