The normal place to do it is within the project settings (in CodeWarrior), under pre-processor. You can add #define MPC5516_CODEWARRIOR. I think you could also place the definition in FreeRTOSConfig.h for the same effect.
I would very much like a copy of your port. Did you base it on the Xilinx PPC port? Could you send it to r (dot) barry =at= freertos.org - thanks.
Of course, I will send you a copy of the port as soon as the port is done. I have to do some small adjustments, but the kernel and an interrupt handler is running on the MPC5516.