Seems like a lot of people are having issues with the amd64 driver from the 2.3 SDK. Here's a workaround:
you should now see your Nexus S listed.
- Go to the PdaNet homepage and download the Windows 7 x64 version of the PC software Download PdaNet
- Make sure your phone is not connected to the computer. Install the app from PdaNet. Ignore the warning about HTC sync if you get it, that doesn't matter for what we're doing.
- At the end of the PdaNet install, you'll be prompted to connect your phone. Make sure USB debugging is on, and connect to a high speed (as in not through a hub) USB port.
- Follow the prompts and accept the unverified driver warning. Let it finish, then reboot your phone into fastboot mode -- power off, then hold volume UP and power.
- Open the device manager in Windows (right click My Computer, properties, choose Device Manager from the list on the left.
- Find the Android 1.0 entry, with the yellow exclamation mark beside it. Right click and choose to update the driver.
- Select to search your computer for the driver manually, and point it at C:\Program Files (x86)\PdaNet for Android\. You will see a warning about an unknown/unsigned driver for sure this time, accept it.