You question isn't too clear.
Do you want your power button to work, or do you want it to "pop" your shut down menu?
In either case, check your BIOS settings to see if your BIOS is "pre-empting" your power key actions, and then look under the "General" tab for the "When the power button is pressed" and see what you have set. (Note: I don't use XBMC. But a search of the XBMC WiKi manual for "power button" turned up that suggestion.)