if your friend is just wanting to join your server, then he just searches for it, there is no mod to enable, its a mission, as soo as the join your server the mission is downloaded. You though have select Play, Multiplayer, click NEW, Type the Name for the server, choose Internet as HOST type, set a password if you only want him/her to join that server, click UPNP and it should setup port forwarding for you, then choose the map Altis or Stratis, then you should see the DUWS in the list for mission, click play and then after its created, they should be able to search for the server name you chose.