To Install the PHP Class Generator Eclipse Plugin simply open your Eclipse Installation and go to the "Install new Software...." Wizard by clicking on "Help > Install New Software..." and click on the "Add" button on the right. In the new window "Add repository" enter a name for the new repository e.g. "PHP Class Generator Plugin" in the field "name" and the plugin update site "http://phpclassgenerator.schallschlucker.de/" in the Location field and click "OK".
Now you should see the Plugin in the in the list in the main window. Just click on "Select All" and click on next. Just follow the Wizard all the way through to finally install the plugin. After this you need to restart the Eclipse and you are ready to go. To learn how to use the Plugin follow the manual on the page "Tutorial: Using the PHP Class Generator Wizard"