This achievement takes place in the second portion of the Dark Carnival campaign. When you arrive at the carnival, look on the right side of the map where there will be a "Peanut Gallery". Score 750 points or more (team members can aid in your score), and a prize box will open on the left side of the stand with Chompski inside. One person must carry him to the end of the campaign and get inside the chopper with him for the achievement to unlock. Here is a video should you prefer it:
Note: This achievement will pop for everyone in the party. Only one person needs to carry the gnome for all to get the achievement.