It seems for now that you can't. The xoom can read the card, but not write to it. You would have to manage putting things on the card by connecting the xoom to a PC and managing it on the desktop.
This is my understanding of the situation, anyway.
I'm expecting some apps will come out soon that find a way around this method. I didn't buy my xoom to hook it to a computer, thats why I didn't get an iPad. I want it to stand alone.
Sent from my DROIDX