1. Connect your iPhone to iTunes via the USB cable to find out your UDID (Unique device id)
In this picture, you can see the UDID is the alpha-numeric string after "Identifer: "
In order to activate your iPhone to allow OS 4.0 Beta firmware installation, you need to retrieve and send us your UDID:
In iTunes, select your device in the "Devices" section and navigate to the Summary tab. Click on the Serial Number label to reveal the Identifier field and the 40 character UDID. Press Command+C to copy the UDID to your clipboard.
In this picture, you can see the UDID is the alpha-numeric string after "Identifer: "
In this picture, you can see the UDID is the alpha-numeric string after "Identifer: "