This has been done under MacOSX 10.6 with the current AndroidSDK. First a little advice: Root first setup all Apps later because the data will be lost during the unlock process.
Step 1 – Software
With all the Software together you are half way there, just install the Android SDK (unzip) somewhere. For me it is in “/Developer/android-sdk”. And run:
$ cd /Developer/android-sdk/tools
And install the newest SDK stuff just to make sure everything recognizes correctly. Extract fastboot and Clockwork and keep the files at hand.
Step 2 – Bootloader unlock
Boot the Nexus S in fastboot mode by holding down “Volume Up” and “Power” to boot. Now your are able to connect via fastboot so run
$ ./fastboot devices
This should show a serial number, which confirms the phone connects correctly. Now it’s time to unlock the bootloader, which will wipe all your data.
$ ./fastboot oem unlock
Step 3 – Flash recovery
Now its time to install a custom recovery which allows for the superuser mod to install correctly.
$ ./fastboot flash recovery recovery-clockwork-220.127.116.11-crespo.img
Step 4 – Install root mod
Finally we are now able to install the Superuser mod. So reboot the phone and copy the su-18.104.22.168-ef-signed.zip to the device. Reboot again into the recovery mode, and run the recovery.
You need to mount system as well as USB storage and now its just a simple install from SD Card to get the rooting going.
Step 5 – All done
Be happy and get some root Apps such as Wireshark :)