Hi severum,
You must set up your profile in the game to enable you to access the internet. (Picture1) You should then see your profile name in the top right hand corner of the screen together with a symbol saying you are connected to the internet.
Then go to "Custom Mission" (Picture 2)
Finally you will find a very long list of custom missions for you to download.(picture 3) Just follow the instructions, but a word of warning, DO NOT try downloading more than one mission at a time as there could well be a mixup between the missions.
You must enter your licence key in your forum profile as well. You can do this by clicking on the word "Profile" in the yellow bar at the top of the screen. Scroll down and you will see a place to enter your licence key.
Kind regards,
Traddles