heart rate monitor question

i recently bought the new balance n4 heart rate monitor.. it works fine when to show my heart rate as i am doing the p90x routines; however, the calories burned stays at 0. i was just wondering if anyone has this heart rate monitor & if so, do i have to press a specific button or something to start tracking my calories or what? because that was the main reason i bought it, so i am a little disappointed for it to not work.