Installer warning FS2004 or FSX isn't installed

It is probably a  register problem.

Visit this page and choose the “FSX/FS9 Registry Utility” 

http://www.tweakfs.com/store/free_tools.php

Or you can also find it on this page under “FSX/FS9 Registry Repair Tool” :

http://www.flight1.com/view.asp?page=library