Upload sketch via xbee

I have two xbee series 2. One is connected to my PC via an xbee explorer, the other to my Arduino Uno via the Arduino Wieless Proto Shield. My Delphi program communicates flawless with the Arduino (sketch) but I cannot upload the sketch from the Arduino IDE into Arduino. I get an error message: "avrdude: stk500_getsync(): not in sync: resp=0x00".

I use the correct COM port. Anyone an idea what I'm doing wrong?