Sorry to hear that - there were some PLX-500's that could possibly exhibit this issue. There is no need to fill out or submit a warranty card to have warranty coverage; you are granted one year of warranty from the date of purchase. What your seller may be referring to is a return policy at their store - some retailers provide an exchange or return within 14 or 30 days.
At this point, you should ask your retailer if they will provide an exchange on a defective unit. If not, you will have to send it to an authorized repair shop for servicing (which is done for free).