How to activate the extension:
1. Go to ~/Library/Safari/Extensions (where "~" means your home folder)
2. Drag "Open in Internet Explorer.safariextz" to your Desktop (effectively moving it out of ~/Library/Safari/Extensions)
3. Double-click "Open in Internet Explorer.safariextz"
Safari should come to the front and ask you if you want to install the extension. If you give your permission, "Open in Internet Explorer.safariextz" will vanish from your Desktop - actually it will be moved (back) to ~/Library/Safari/Extensions, BUT it will as well be added to the list of installed Safari extensions that resides in an XML-file "~/Library/Safari/Extensions/Extensions.plist".
Dear Parallels-install-package programmers:
Simply moving an extension file to ~/Library/Safari/Extensions does NOT activate it in Safari.
I only accidentally stumbled upon this thread because I wondered why I have a "OpenIE.safariextz" in my extensions folder while transferring stuff to a new computer – otherwise I would never have know that there now is such a neat extension in Parallels Desktop 8 (I use Parallels Desktop since version 2), since it was sleeping unactivated.
Regards