Hi Sushigal,
Your right, I downloaded the "Category Listing Box" Add-on and there aren't any installation instructions.
To install this Add-on, you just need to upload the files to the /includes/ folder on your site. There are a few folders called YOUR_TEMPLATE where you need to replace "YOUR_TEMPLATE" with the folder name of your template.
* Note that whenever you install a module, you want to make sure that you aren't over-writing any of your custom changes to your store.
Once you have upload these files, the Add-on will be installed and you can turn it on from Admin->Layout Boxes Controller.
- Danny



