So here is the steps to unlock Google Nexus S Bootloader:
- Turn your phone off
- Hold down volume up + power together at the same time
- As the result, you will in the standard recovery mode now
- Let's say you already installed Android 2.3 Gingerbread SDK with fastboot, now on terminal/windows or Windows cmd depending on OS type "fastboot devices" to check your device can be seem via USB
- Assuming you device is seen via fastboot devices, now type "fastboot oem unlock"
- Accept and well done and now your just purchased Nexus S bootloader is unlocked.
Via: Android Centeral.