can you send a copy of the page that has the add to c art button on it so i can see the code?
Also a link where i can see the problem would be helpful.
There is no automated process for adding the new column to the order details table, you will need to use PHPMyAdmin or another mySQL GUI tool to access the database, edit the order details table to add the additional column.
Also, you will need to edit the store order details behavior on the confirm page to bind the new column to the RopeLength column of the cart.