Arduino BT  Resource busy

I'm trying to Upload on my brand new Arduino BT some sketches that worked perfectly on my Arduino 2009. When I launch Upload, I receive on the Monitor the red message: ser_open(): can't open device "/dev/tty/.Bluetooth-Modem": Resource busy. In Tools, the Board is set on Arduino BT and Serial Port is set on Bluetooth-Modem, but I have tried also the other three options for Serial Port without success. The board is connected to a 5V power supplier and only the green LED, close to 3V3, is ON. I'm working with a Mac G5 with OSX 10.5.8. Any help?