i would bring the machien back to the dealer. as you had said yesterday...sewed perfectly at the dealer's, then when you got it home, played with the settings, and now having a problem. i would ask the dealer to help you operate the machine correctly. and if not able, to return it. i would also do this NOW and not later. while his memory is still fresh about the sale....
i would also ask your parent(s) or grandma to go with you. sometimes having an adult back you up is a good thing.