Step 1: Download the add-on.
All downloads are in .zip format. (You need to open the .zip archive to access the .vpk file.)
If you can not open or extract the .zip/.rar file, download and install winrar for free.

Step 2: Install the add-on.
Copy the .vpk to your Alien Swarm add-on directory. If this folder does not exist, create this folder.
\Steam\steamapps\common\alien swarm\swarm\addons\

You can remove add-ons by deleting them from your /addons/ folder.