I have installed a freeware version of KPBI, but GSX isn't recognizing the afcad. I checked and it's a full ADEX file but GSX is still looking for the stock afcad, which I have renamed as a backup file.
First, be sure you have the current version of the software, which can be obtained by downloading and installing the latest Stand-Alone Addon Manager.
Once you are sure you have the latest version, which is supposed to detect changes to your Scenery Library more reliably, try to use the "Restart Couatl and rebuild airport cache" option in the "Couatl - Settings" menu.
Assuming you *are* using the latest version of the software, and the problem still can only be fixed by manually rebuilding the airport cache, it's possible the issue happened because you renamed the original file.
There's no need to do that and, in fact, you shouldn't do that, since the freeware scenery might NEED to elements of the default file. You only need to place the freeware scenery on an HIGHER layer in the Scenery Library, and GSX will surely use it.
If the scenery is still not recognized, even after rebuilding the airports cache, then it's possible GSX might have rejected it for some reason. Enabling logging in the Troubleshooting section of the GSX - Setting and doing a another rebuild, will have any reasons why that particular file has been rejected in the Couatl.LOG file that will be created under the %APPDATA%\Virtuali folder, if you enable Logging.