I will try to describe it much more detailed :
1.Press and hold the reset button then power on the device.
2.Check if the DIAG LED is fast blinking and it is in loader mode.
3.tftp the file that you found on this page http://www.cpx.cz/dls/WP54AG_linux/WP54_MSSID_AK_V322_B090629.ZIP
   tftp -i 192.168.168.1 put <filename>
4.Continue to observe DIAG LED, it will be lighting up if it is loading the firmware into the device.
5.DIAG LED will blink slowly when firmware is load complete. You must wait till this !!
6.Reboot the device.