If you are using an IRTrans USB module, you need a "driver" to make the OS recognize the module. Please download this package: "FTDIUSBSerialDriver_v2_2_16.dmg" (434 KB)
After downloading and double-clicking the package, this window shows up:

Double click the right package and you will be guided through the installation. It is recommended to quit iRed and unplug the USB module before installation and restart the Mac after installation!
This driver was tested on Mac OS X 10.5 (Leopard), 10.6 (Snow Leopard), 10.7 (Lion) and 10.7 64-bit.
Please refrain from looking for newer drivers on the Internet, because only the provided driver is tested for use with iRed2.
Please see the Troubleshooting page, if you encounter problems.