I think that in this case there is no PDE (plugin development environment), Window Maker probably depends on this for its part of SWT / JFace.
You can try adding the full PDE package from the eclipse software site. In Eclipse, go to: Help - Install New Software - Work with: http://download.eclipse.org/releases/helios
After a long update, the "General Purpose" node should appear in the list. After expanding the node, you should see the "Eclipse Plug-in Development Environment". Try installing this first and then Window Maker Pro.
Alternatively, you can always use Yoxos - http://ondemand.yoxos.com/geteclipse/start - to fully customize your Eclipse installation.
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