Design view of Eclipse windowbuilder pugin on Windows 8.1 x64 prompts "Unknown GUI toolkit"

You don't need to reinstall the whole WindowBuilder. Your toolkit was probably SWT, and SWT designer might not come included in your eclipse download. All you need to do is to add SWT designer features to your WindowBuilder from "Install New Software" menu, selecting default Luna installation site, searching for "SWT Designer".

I fixed the problem by:

1. Unistalling the window builder
2. Downloading the zip package of the new version of the window builder from here.
3. Then I extracted that and put its extracted directory inside dropins folder of eclipse.
4. Then restarted eclipse.

And WindowBuilder worked fine.