If needed, the Flashtool package has the capability to unlock your bootloader. Be warned, this process will modify the part of your phone which holds the DRM keys to Sony's Xperia services and you will lose access to them. It is best to first back it up as the keys are unique to your phone.
Here's a link to my guide to backing up your Sony Xperia DRM keys.
Once you've got Flashtool:
- Install and run it.
- Turn off the phone
- Unplug it from the computer
- Click on the "BLU" icon in flashtool
- Hold the VOLUME DOWN button and plug in the phone to get into flash mode
- The indicator LED should be green
- Let flashtool detect and do it's thing
- Unplug the phone
- Hold the VOLUME UP button and plug in the phone to get into fastboot mode
- Indiciator LED should now be blue
- Follow the instructions in the "Bootloader unlock wizard"