Mac Gaming Mods

I just started playing this, but when I click on Campaign, it takes me to the difficulty selection, and then to the Silent Cartographer mission. I want to start the game from the beginning. What's going on?

It's only the demo version of halo which only comes with the silent cartographer mission. If you want to play the entire campaign you need to get the map files from the full version of halo and put them in your maps folder.