Do What This Guy Does
Download files in his description.
Now after watching the video I'm not sure what I did but here is what I know.
You need to have access (be white listed to no pixel) to steal their server side files (which means if you want to be able to interact with shops, rob peope, etc. you need the server files).
{{If you don't care about that you just download the files and host your game just to play on the map.}}
Once playing on their Official Server the files should be saved in a folder as shown in the video. You need to view the files in PBO Manger and copy and paste to a folder. Then open PBO View and pack the folder you just saved the files to. Rename the new .pbo and put it in Arma 3 MPMissions Folder. Then you should be able to Host it like normal.
This probably sounds very confusing and makes no sense. So just message what you need help on. Sorry first tutorial. Good Luck!