If you've changed the plug from the kit supplied item you'll notice that the new plug has a larger diameter top than the one that came with your engine. That cap will unscrew and then the plug boot will snap on. Check your spark plug. The kit plug boot is made to fit a plug with some threads on the top. It will not fit a standard plug with the cap.
My advice is to get rid of the plastic plug boot and replace it with a good quality automotive grade item. In case you haven't noticed yet; the plug wire screws into the plug boot as well as the CDI.
Tom