Unplug the card, than plug it back in and do "dmesg | tail". It should read something like this:
root@debian:~# dmesg | tail -n 5
mmc0: card bb41 removed
mmc0: host does not support reading read-only switch. assuming write-enable.
mmc0: new SD card at address bb41
mmcblk0: mmc0:bb41 SU02G 1.89 GiB
If it does, the card is being recognized by your kernel. You should now have a device called /dev/mmcblk0. If you don't, you should check either your card, your kernel or your modules.