Hi n9mfk9,
When you start the game you will see the screen as in the first picture below. If you click on "Download content" you will find a long list of custom missions. Click on the one you want to download and the download button at the bottom left of the screen. Once downloaded a mission will appear in your "Single mission" list when you click on "Play", see first picture again.
If you click on play you will find a list of "Campaign" missions each of which has (I think) six missins included. See picture 2.
Regards,
Traddles.
