It's found in the multiplayer part of Arma. I'm guessing you are looking in the single player area.

If you haven't launch the multiplayer browser, here's how:
Launch Arma. Once it's started select 'multiplayer' then 'server browser'
At the top select 'host server'. A list of settings will show up. Look at the 'HOST' option. If you have it set to 'LAN' then it will be a private game. Click the 'Host Server' button.
Next you can choose the map on the left, and the mission on the right. So click 'Khe Sanh' on the left and you should see 'Ground Patrol' on the right. Click it then click 'play'.

hello i can't fire up the mission as I can't find the module in the editor or i can't find a scenario or a game start in the menu, thanks in advance