Arduino wireless SD shield

Hi Forum,

I wish to send data from a remote sensor to the Arduino wireless SD shield atop a UNO. The rig works fine, logging data to the shield, but only if the coordinator Xbee (base) is overboard of the shield, that is, only if the base Xbee is on an adapter on a breadboard, with RX-TX wires to the shield.

I've looked in many places unsuccessfully to find out what the trick is for using the Xbee on the shield itself. Can anyone clue me in? :)