This is the wrong forum for X2 issues but since you asked, X2 Producer does not include anything called a song construction kit. It does come with a 4GB+ sound library called "Loops and One Shots" which has a separate installer. If you had a Cakewalk product that had a song construction kit and want the kit you will need to install that product. The X series forum is here.

Hi Scook
You have cleared this mystery for me.
I have music creator 6 still installed on my laptop which includes the song construction kits. Music creator 6 is not installed on my main computer. I went round to my friends this evening and he had the kits on his computer with sonar x3 so we both presumed they were included. I have just sent him an email telling him the reason.
I know he has music creator 6 on his computer as well.
Thank you for clearing this up for me. I have been scratching my head about it all day.
Sandra