The LED is actually connected to the chip itself. When the receiver receives a data from the sender, the receiver will light up this LED. Only when this LED is on, then the user is allowed to press a button to send a reply.

The LED is connected to Port A pin 0
The button is connected to Port A pin 1
The Receive pin is Port B pin 0
The Sending pin is Port B pin 1
The carrier detect pin (from the RF transceiver which will switch to low when a signal is detected) is Port B pin 2

My program looks roughly like this:
DEFINE OSC 20
DEFINE SER2_BITS 9
REC VAR BYTE
DATA VAR BYTE
TRISA=%00000010
TRISB=%00000001
TRISC=$00

RECEIVE: IF PORTB.2=0 THEN
SERIN2 PORTB.0,8380,RECEIVE,[WAIT("TUV"),REC]
HIGH PORTA.0
ENDIF
REPLY: IF (PORTA.0 = 1) AND (PORTA.1 = 0) THEN
SEROUT2 PORTB.1,8380,["TUV",PRESS]
LOW PORTA.0
ENDIF
END

This is actually a small part of my whole program. The finished receiver should be able to take in message from 3 senders.