I don't have the Horizon but thread shredding usually means bad
needle...either wrong size or just a bad needle. To check if the needle
is the right size for your thread, hold the needle in your hand and thread
it then hold each end of the thread and see if it slides. If it sticks
you'll know you need a needle with a slightly bigger hole.
Hope you can sort this out. I wouldn't give my business to that dealer again
if he is so unwilling to help.