Do never add custom content to the default addons folder. We have installation instructions on 99% on all our downloadpages:
As we always recommend use modfolders to seperate the custom content from the official game content to prevent problems. For different ways to set up your modfolders and use them please visit our FAQ.
When you are using the Steam version you can find a Steam mod installation and activation FAQ here.
Read the FAQ and make sure your using an equal system.
It should not matter wether you use winrar or 7z, they can both work with any of the files we host. Make sure to grab the latest version if you are experiencing troubles.