Mount the standard freelancer ISO that you downloaded from the download site that we use on this forum.
Install Freelancer, in the standard path.
Install discovery, first the game will ask you for the Standard freelancer path, after that it asks you where you want to install it, don't use the freelancer folder and install it where you want.
If that doesn't work, post here again.
Ps. Win 10 can mount ISO's without any problems, it's a windows feature so you don't need any other tools.