Unfortunately you've bought a seat that does not fit the type of seatpost you have. To mount that seat to your bike you will need to purchase a new seatpost. What you are looking for is what is called a pin type seat post. Look around the bottom 2 ins. of your old seatpost for the size it will be a number like 26.4, 26.6, 26.8 or larger this number is the diameter in millimeters. Then phone around to the bike shops in your area to see if anyone has one in the size you need. These types of posts aren't very expensive (approx. $10.00).

ocscully