Have you got pull up resistors?
yes, offcourse.
Are you using the analogue pins 4 & 5 not the digital ones?
Im use on the arduino uno the pins located near the reset button, marked with SDA/SCL. so not use pin 3 and 4.
Have you got the anode of the LED connected to 5V, cathode to a resistor and resistor to the PCF output pin?
yes, i hope the attachment is here u can see the diagram i quickly made.
i have also tried some other LED's with same voltage and less mA. but same result.
