Hi,
I have a wavecom fastrack GSM modem that has only a USB terminal. In order to connect it to the arduino I have used this converter (http://robokits.co.in/shop/index.php?main_page=product_info&cPath=6_64&products_id=64) . I connected the Tx and Rx of the converter to the Tx and Rx of the arduino. Connected the +5 and gnd wires of the converter to the corresponding terminals on the Uno. then hooked up the modem to the converter. Is this right???
The converter you bought turns a PC USB host port into a serial port. It won't work to talk to your modem. You need something like a USB Host Shield on your Arduino to allow USB devices to connect to the Arduino instead of a PC.