When you meet up with the French rebels you will have a choice to go left to the Flak 88s or go through the house to the right. Go left to the guns, then kill all the Nazis around the gun and in the ditch leading to the next gun. Then, blow up the 88 and move on to the rest of the guns. After you blow up all the guns you will reach the house you would have if you went right. When you go in the house you must defeat a Nazi that is standing there waiting for you. Kill all the enemies in the house and fall through the burning floor. You will see your allies there waiting for you.