Installation instructions can be found on the GitHub page.
4. Download the addon from GmodStore
Click "Download Stable" on the addon's GmodStore page
5. Install the addon
Move the folder(s) which are inside the .zip you just downloaded from GmodStore into your server's garrysmod/addons/ directory (including the xeon-drm folder)
6. Restart your server
(If it was running already)
7. Activate your licenses
Join the server and follow the on-screen instructions - if you've already done this before, you probably won't need to do this again.