It appeared that there's no automatic driver installation for Google Nexus S under Windows 7 x64. Even after installing Andoid SDK and downloading drivers, Flash Builder 4.5 not recognizes it as valid device.
In Device Manager Google Nexus S will be displayed with yellow triangle sign as it were unrecognized device. Right-click it and select Properties. Copy-paste Hardware ID information. As written in this Adobe article, edit <Adobe Flash Builder 4.5 Home>\utilities\drivers\android\android_winusb.inf by adding strings like this into [Google.NTx86] section:
;Google Nexus S %CompositeAdbInterface% = USB_Install, USB\VID_18D1&PID_4E22&REV_0227&MI_01 %CompositeAdbInterface% = USB_Install, USB\VID_18D1&PID_4E22&MI_01
Then 'Update driver' for Nexus S unrecognized device using <Adobe Flash Builder 4.5 Home>\utilities\drivers\android\android_winusb.inf.
Check out other ideas on this great page.