If fastboot from your computer can't find it, try following these steps exactly, beginning with installing the
Requirements below. The most likely problem is some sort of driver missing, disallowing the computer from reading the phone in while it's in the bootloader.
This guide can be found on addictivetips.com, and uses a fastboot program from your computer to unlock the bootloader:
- To begin, navigate to this website: http://unlockbootloader.sonyericsson.com/instructions.
- Click Continue at the bottom of the page and enter your device’s IMEI code, name and email address in the relevant fields.
- Your unique unlock key will now be mailed to you.
- Now unzip fastboot.rar and open the folder extracted.
- Within that folder, hold the Shift key and press the right Mouse button. Now select Open command window here.
- Now we need to connect the phone to the pc in fastboot mode. Do so by turning the phone off, holding the menu button (search button for the Xperia Play) and plugging in the USB Cable.
- Now in the command prompt, enter the following command:
fastboot.exe -i 0x0fce getvar version
- Now enter the command, replacing KEY in the string with the unlock key mailed to you:
fastboot.exe -i 0x0fce oem unlock 0xKEY
That is it! The bootloader is now unlocked.