Step 1: Power off your Nexus S completely.
Step 2: Now hold the Volume Up key and the Power key simultaneously.
Step 3: You should now be in standard recovery mode.
Step 4: At this point, you must have the Android 2.3 SDK installed with Fastboot. If you’re in Windows, open the Command Prompt or if you’re on a Mac, open the Terminal app then type “fastboot devices”. This will tell you whether you’re connected via USB or not.
Step 5: If you’re connected and fastboot is showing you as connected, type “fastboot oem unlock”.
Step 6: Accept and that’s it, you’re done, your Nexus S’ bootloader is now unlocked.
If you want relock bootloader
Step 7: If you’re connected and fastboot is showing you as connected, type “fastboot oem lock”.
