The first step is to make sure you have the addon. I know it's obvious, but make sure you have the correct and official addon: https://steamcommunity.com/sharedfiles/filedetails/?id=2916561591
Once you have the addon subscribed you should make sure it is enabled. You can do that by launching Garry's Mod, clicking the Addons button, and finding our addon.
Once you confirm the above information, launch a Singleplayer or Local Server and let the installation complete. You can do so by clicking Start New Game, picking a map, and proceeding.
If you want to confirm it worked and don't mind some extra work yourself, you can actually navigate to your Garrys Mod/garrysmod/data/wos/dynabase folder and see the new data files made!