Hi, I'm also having this same issue. I am on the standalone version 2.8.6 open beta, and I cant get it to show up in the module manager.
Edit: I see that i am actually currently on 2.4. version and not 2.8. Please check your game version before posting about not being able to download the Sinai map! Also keep in mind the OPEN BETA version of the game is a "separate" game, so you have to open the open beta version of the game to play the Sinai map (Currently)