I found this project but I do not know how to download and install.

ps. I can not add a link to this forum.

You may have already figured it out yourself, but Redux isn't a mod, it's just a UI design project. You can't really download it because it just doesn't exist.

And yes it is normal that you can not post (yet)

You need to have a rep of +5, it is GOG's way of countering SPAM-bots.

For every day you post, you get +1 rep, if you help others with a problem, so a thread marked as question (yellow point instead of a green point) and your answer is chosen as the solution, you get +5