This site explaing how to use an old Floppy disk connector to ready SD/MMC:
http://uanr.com/sdfloppy/
This other site presents a basic about accessing MMC:
http://www.retroleum.co.uk/electronics-articles/basic-mmc-card-access/
And finally this site presents a circuit and program to access SD/MMC card over SPI:
http://www.captain.at/electronic-atmega-mmc.php