This site is being deprecated.
Please see the official X‑Plane Support page for help.
It sounds like the installer is not able to write to the DRM file. You can check the permissions on the file in the path listed (some folders may be hidden by default) or even delete the file entirely. X-Plane will rewrite it the next time you launch the sim or need it for the installer.
It sounds like you are either missing steps or are not connected to the internet. After you pick the option to install a product purchase, the next screen should ask you for your digital download product key or dvd 1. After that it should try to validate your key, offer a screen to change the name or location of the install folder (we do NOT recommend installing to C: or Program Files), and a screen to change how much scenery is installed. Here are step by step guidelines on installing.
This site is no longer being actively maintained.