To reach the Hurtgen, Rocket and Berlin missions in the Campaign, you first need to complete the American, British and Russian Missions.
You can play on any difficulty, but if you're not very good at the game, play on recruit. This will be the easiest difficulty. Your health does not regenerate, you have to pick up health packs. Besides, you can not sprint. If you need any help with this mission, there's a great guide on youtube by labradoodlesrule:
Also playthroughs of other missions can be found on his youtube account.