Look inside the square hole. You should see the end of two cotter pins coming in from opposing directions. Straighten the legs and pull the pins out from the outside. The force you previously applied may have deformed the cotters. If you can't pull the pins out readily, try tightening the nut and then pull out the pins. A 3/8" socket extension should fit the nut.
As I recall, the prodcedure is spelled out on the Martec website.
