This is the method I used for flashing the radio.

Method 2 (FASTBOOT + ADB):
(FOR WINDOWS Operation System only, NOT FOR MAC OS)
Part 1
1, Download Radio Flash (Fastboot&ADB).zip
2, Extract the folder to Desktop.
3, Open the extracted flash folder.

Part 2
1, Download the radio.zip file you would like to flash from the link above/attachment below.
2, Extract it to Desktop.
3, Copy the "radio.img" file from the folder and paste it into the Radio Flash Folder you had extracted in PART 1.

Part 3
1, Connect your Nexus S to computer (USB Debugging turn on, SD card mount turn off)
2, Double left click the "START HERE.bat" and command prompt window will open/run.
3, Type: "adb reboot bootloader". (The adb reboot bootloader will take the device directly to fastboot) OR boot the phone to bootloader by pressing the volume up key with power key at the same time.
4, Once your phone has rebooted in to bootloader screen, type "fastboot devices" to ensure your device is detected.
5, After confirm your device is detected with serial number, type "fastboot flash radio radio.img".
6, Once its done, type "fastboot reboot".
7, You're good to go