help , fastboot problem xperia arc

Hello .. my phone is Xperia arc lt15i and i wanted to install android 4 on it but i stuck with the first step ..first (bootloader allowed true)
but i can't enter the fastboot .. i turn of my phone and then press and hold the menu key and plug the cable and the blue light turns on for maybe 0.1 sec and then off and then starts charging ... i heard that if the blue light did't turn on then i can't enter fastboot at all .. but my blue light turns on ,but off again ..so it's not that case(right?) .. can anyone help me please