Hi, have a look in your instruction book to find the largest size of memory card your camera can use. If you have bought one that is too large the camera will not be able to use it, therefore it will say the card is unusable.
E.g. largest card camera can use is say 2gb, you accidently buy one of say 4gb. The 2gb card uses up to 16 bit information, but the 4gb card uses up to 32 bit information. Therefore theb 4gb card is unusable in the lower spec camera.
Solution: buy smaller capacity cards, which will work.
Hope this helps, if not please get back in touch.
Regards.
John Bates, December 2008