PC  Xbee  Another Xbee Communication
Basically with your shield you have two options:


Communicate between the MCU and the XBee at the cost of not being able to communicate between the PC and the MCU.
Communicate between the PC and the XBee at the cost of having to remove the MCU from the board completely.


Neither of which are particularly ideal.  The first option is completely out since that rules out all PC communication.  The second option turns the Arduino into just a USB->TTL Serial adapter - and a rather expensive one at that (you can get them on eBay for like $2).

What you want is to re-wire the system using jumper wires (messy, I know) so that the TX and RX pins of the XBee connect to other pins on your Uno then use SoftwareSerial to communicate with the XBee.  That will leave the Arduino's TX and RX pins free for communicating with the PC.

Alternatively invest in a different Arduino that has more hardware UART interfaces, like a Mega2560.