Interruptions help

Basicaly i have this code that handle a del light. To change the state of the del i use an interruption. But the thing is no matter if i press the button to turn off the light it does not work.
I think that may be it is due to the way i initiate the interruption.
here the code


ISR(INT0_vect)
{
 // here i put the code to change the state, it is just an if else 
 
}

i have to use MCUCR register does any one know how it works? i mean if i need to assign it a value what is the best way to do it?