The sternum or breast bone is a long, flat bone in the front of the chest. The ribs (most of them) attach to the sternum with cartilage at sites called costochondral junctions. Separation of a rib from the sternum such as you have experienced is called a costochondral separation and can occur as the result of a sports injury, usually due to excessive twisting of the trunk. As you undoubtedly know, this can be very painful, especially when you breathe deeply, cough or sneeze.
As with other injuries involving the ribs, the most important thing you can do is relieve the pain so you can breathe normally. Unfortunately, this particular injury can take two to three months to heal, up to twice as long as for a rib fracture. Try to rest and try to avoid movement that causes pain. The Marine medics may have taped your chest to reduce movement. If not, you might request it. While you're healing - and afterward - do what you can to avoid putting any stress on the area or engaging in any activity that might cause further injury. Once healing is complete, you might consider wearing some type of protective padding - something similar to the flak jackets some football players wear - to protect your chest area when you're engaging in activities that might expose you to re-injury.
I would recommend taking vitamin C - maybe 250 milligrams - twice a day for a month to help the body make new connective tissue. Also try topical application of Traumeel Cream, a homeopathic preparation that relieves pain and stimulates healing of areas of injury, especially joints.
Dr. Andrew Weil