This is from the Werner Ladder Co. FAQ for wood folding attic ladders:
For reattaching the springs on your model ladder you will need two people. One person will need to go into the attic and have the door closed. Once they are in overhead space and the person below is holding the ladder closed the springs will be able to carefully be removed. Please contact our customer care department at 888-523-3370 Monday through Friday 8 AM till 5 PM EST if you have any questions, or need any additional information
So... it seems to me that @keshlam's comment is correct, you need to use brute force. Get two people in the attic to help stretch the springs if you need to, while the door is held closed from below by a person standing out of the swing radius of the door and propping it up with a 2 x 4.
Or, using the above prescribed procedure for holding the door closed, have the person in the attic attach a "come-along" to the end loop of the spring and a sturdy anchor point. The spring could be easily stretched in this manner. Or fashion a block & tackle using some rope and a couple pulleys; it should be fairly easy to rig up a mechanical advantage for this.