I would not bother with the missions, personally I would Google tutorials on how to use the Mission Editor. I wish somebody had told me years ago, it is very easy once you grasp the basic controls - this is the reason why Operation Flashpoint & Arma have such a loyal fan-base despite the bugs.
You can literally place a few units and play your own mission. I messed about the other night and created a mission (taking 5 mins to do so) where the invading army took over a town, I was holed up in a building defending, and it was brilliant to the point where I had to avoid windows as the enemy will shoot through etc. It is great fun and the mission does not play the same every time!
Once you get used to the controls, which in comparison to traditional FPS are slow, nevertheless, it is a great game, but you do have to persevere - Have fun!