If your problem is simply getting the Nero burn engine to be used as the ‘engine of choice’ in DVDFab, click on the check-mark of the opening screen when you start DVDFab,
This will take you to the Common Settings screen. Select DVD - Write.
Your first option here will be burning engine. It should, by programmed default, be on VSO burn engine. Click the little arrow to the right of that selection, and you should see three options: VSO, Nero, and IMGBurn.
Select Nero, exit Common Settings (DVDFab will re-start) and you should be all set.
If you need to find the file manually, look in the Ahead directory in Program Files, and look for the WMPBurn sub-folder. There will be two files in that, the burn EXE file and the DLL file Service library (at least, that’s what I have in my Nero 7 set-up).