Hello sir,
Sorry for the inconvenience. Kindly check your Windows Regional Settings in your Control Panel. The Decimal symbol needs to be a "." and the Digit grouping symbol needs to be a "," as shown in the attached image, then try again.
You get the .NET Error most probably becase you don't have P3D v3 installed yet.
If you are intending to migrate from 2.5 to 3, please read the migration text document in the start menu before starting the process. Please contact us if you feel that you need any help or clarification.
I have sent you this reply by email as well.
Awaiting your feedback