There is a small subset of the most basic missions which are included in the game when you download it. In the main menu, go to singleplayer->missions to play them.
Other missions, including the ones from @KingRaptor old
campaign which you are probably referring to, are not all being maintained and may be replaced by a more simplistic campaign in the future. If you absolutely want to play them, you may get some help in-game, but it will require some tinkering to work.