Hello,
When you press "Flash" button for X500 flashing, the LED don't start to blink. It's Ok.
Just press and hold some time phone "Power" button and flashing process should start.
If you still have problems, please, check your cable.
Here is link with info:
http://forum.gsmhosting.com/vbb/showthread.php?t=211600
B.R.
InfinitySupport