The web site with only a description (no schematic) is http://www.seeedstudio.com/depot/sd-card-shield-p-492.html
There is no description of the connectors to this SD shield. It seems to me that a 6 pin connector on the arduino due (next to the processor chip) needs to be connected to the connectors on the SD shield. I don't know the name of these and at the store they couldn't help. The 5V vs 3.3 V problem you mention could be solved with a jumper from the Due to the SD shield. Any ideas on the connectors and how to connect them?
MJM