weg said:You should have done that long ago: Send them a written notice (registered) that you give them a (reasonable, i.e., at least 2 weeks) deadline to deliver your iMac, and that you want them to give you your money back if they are unable to comply.
That letter is worth less than even the registered post you sent for it, when you paid your deposit you signed a sales agreement, don't like it? Just walk, and oh, the deposit is no longer yours too...
Paying a deposit for anything is really *not wise*