So - let's check first:
a) Do you have a CHDK compatible version. What does your cam report ?
b) Are you sure you properly made your card bootable ? Even if you did,
is it a new card to you ? Some cards will just absolutely refuse to be
made bootable, they block access to 1st sector. To check out card,
I would advise testing it's functionality in your s3is first - assuming
you have CHDK experience with it.