Here you find some hints for troubleshooting.
The IRTrans USB module needs a driver to be recognized by Mac OS X. Go to the IRTrans USB page to see how to install this driver.
Please refrain from looking for newer drivers on the Internet, because only the provided driver is tested for use with iRed2.
To check whether the USB driver is installed correctly, please perform the following steps:
This screenshot of System Profiler shows my configuration:

Unfortunately, the USB driver will not load if your system is booting in 64-bit mode. This is the default mode on most Snow Leopard Server installations
To check, whether your system runs in 64-bit, open the System Profiler, then select the "Software" entry:

My iMac is running shows, that it is NOT running in 64-bit.
Unless FTDI releases a 64-bit version of this driver, there is no solution for this problem. While Mac OS X can run 64- and 32-bit apps side-by-side, this is not possible for so-called "kernel extensions" like drivers.
However, you can boot into 32-bit mode! I am not sure whether the performance will suffer by this measure, but you can give it a try. There are some methods to force 32-bit mode, one is the free tool "Startup Mode Selector" by Markus Winter. Please see his website for download and for more info about this 64/32 bit mode magic.
To try out 32-bit mode, you can hold "3" and "2" while your Mac is booting! This setting will not be stored, so if you encounter problems, simply boot again and you are back on your default settings!