If you are building the project using the CodeWarrior wizard, the wizard should have generated a processor-specific header file that contains the mappings of variable names to peripheral registers, which in turn drive the pins. Look for a header file the uses the CPU's name or board's name, say Lite5200.h. In there you shoud find the variable name IRDA_RX, and what address that is associated with it. To write to the pin, you have to set it as an output (if it is a general-purpose I/O pin that could be an input or an output), then write a bit using IRDA_RX as the reference to the register.
Look at some of the example programs that should have come with the board's CD. There is usually a simple example program that blinks the LEDs that you can study and learn.