This achievement IS missable! If you do not hit the jump during the 2nd part of the first mission you need to Reload Checkpoint. If you beat the mission you need to load an old save as Saints Row The Third does not have a way to replay missions.
When you Jump from the Helicopter push forward on the left stick then before you get too close release your parachute and aim over your landing zone and pull back on the stick. Achievement pops when you touch ground.
EDIT: Only achievement you have to earn for yourself in this DLC. All others will pop for both players in Co-Op