The spool holder is sitting on top of a regular spool holder with the loop at the top, so the thread goes directly up to that loop, then it goes to the machine. That top loop is not shown in the picture.
The key to getting it to work on the 15 is putting a piece of tape across the top thread guide on the back face plate, turning it into a hole instead of the open notch. If you do not do this, the thread will pop out and you'll have Problems!
However it threads exactly the same, that is just to keep the thread in place. I'll take some pictures when I get home after work (am in the office today).