You don't mention what software stack you are using... Simple MAC, 802.15.4MAC, or ZigBee BeeStack.
 
If you are using ZigBee BeeStack use this:
void Asp_PortReq(uint8_t portWrite, uint8_t *portValue);
 
If not, you need to write the values directly to the ABEL registers 0x3F.  See MC1321xRM (reference manual).
 
 
NOTE: Don't foget to set the DDR (Data Direction Register) before hand (during initialization).
 
If you are using ZigBee BeeStack use this:
void    Asp_DdrReq(uint8_t directionMask);
 
If not, you need to write the values directly to the ABEL registers 0x0B.  See MC1321xRM (reference manual).
 
 
WARNING:  Messing with the radio's GPIO can cause big problems if you are not careful to avoid the I/O used to interface to the main processor.  See MC1321xRM (reference manual).
 
Regards,
 - Ware