It sounds like you may have a bad card (I can't be sure from the info you have given, but it does sound like you have a compatible card - it may just be a problem with your particular card). You may want to check the list of compatible cards, and try another card from the list. You will also want to double check that the card holders pins are aligned and making good contact with the card. You may also want to make sureshould verify that you are supplying enough power to your pi.