So you probably already know this, back in November 2018, Valve has released an update that disabled all addons in all versus game-modes in every Official Dedicated server they run. This means that you'll be playing the gamemode without your favorite mods installed

1. Download the mod first by subscribing to the addon, if you already did then just skip this step.
   
2. Go to your workshop addons folder, its located in left4dead2/addons/workshop
   
3. Find '1206291369.vpk' and drag it outside to your Left 4 Dead 2 folder.
   
4. On your Left 4 Dead 2 installation folder, create a new folder and then rename it to 'reworked_hud' (This can be any name, as long as you replace space with '_')
   
5. Put the vpk file inside the folder and then rename the vpk file to 'pak01_dir'
   
6. Now go back and into your left4dead2 folder and open 'gameinfo.txt'
   
7. Scroll down all the way until you find the SearchPath line, this is where you'll need to mount the folder you created earlier.
   
8. Copy the line 'Game update' and then paste it above the line itself, then replace 'update' with 'reworked_hud' or whatever you named the folder earlier.
   
9. Save 'gameinfo.txt' and then launch the game

If the addon shows up when you play a map without loading your mods, then congratulations, you just successfully mounted the addon. If not, then I suggest you take a look again at what you may have missed.