Arduino Mega sleep and wake up


I am referring to this website for the following Arduino, Zigbee and Embedded Development: Sleeping Arduino - Part 2 Wake Up Via An External Interrupt

The Arduino goes to sleep after three seconds as it should. However, i cannot get it to wake up when i click a button (external interrupt). The code doesn't return to the loop function to continue the process. I know the button is working fine because i did some simple text print tests.

Can anybody see a problem with this?

Thank you