When having troubles with a USB-to-Serial converter or adapter, please follow these steps:
(1) Make sure the appropriate drivers are loaded and that the module is recognized by the Operating System. In windows it will show-up as a COM port and in Linux as a device /dev/ttyUSB0 for instance. The most common drivers can be found here: ftdichip.com/FTDrivers.htm
(2) Once the module is properly installed, do a Loop-back test. This test consists in connecting the RX line to the TX line and sending messages through a serial communication program such as Hyperterminal in Windows. The messages should immediately come back since the lines are connected together.
(3) If there are still problems and there are no shorts or power supply issues, then it is likely the interface is defective and that you will need to exchange it. Please provide your invoice number and product code when requesting an exchange.