i think i understand your problem, it seems to be the same one as i have : are you trying to use a usb memory card reader to read the card on your pc ? i bought one and the pc does not recognize the card, it does not see anything in the reader at all and gives me the message "please insert card" as well, however all the books i transferred to the card are visible on the reader (and other types of memory cards are recognized and accessible by the pc via the card reader, so it's very strange).
i do not know where this problem comes from but
Stephanos has suggested a way around it which does not use the card reader, see
this thread.