You must
- set the ex0 and ea flags in the interrupt enable register
- you may want to select edge triggered / level activated action by setting the it0 bit in the timer control register
- Make sure the /int0 port pin is high, no other part of your program should ever force it low, or it woun't work.

Please be more specific when you ask questions! I don't know what hardware you have connected to the e0 pin, I don't know what your program looks like, and I don't know what you have diagnosed so far yourself...