How to get your device's UDID
To take part in the beta program, it is required to send me the UDID of your iPhone or iPod Touch, where the app should run on. Distribution of apps for iPhone OS is restricted to Apple's App Store. To make beta testing possible, Apple has created the "Ad Hoc Distribution" model, which is based on the UDID.
The UDID is the "Unique Device Identifier", a 40 character hex number, which looks like this fake example: 7915757d5c5766abe2848731ce10772cbebcac30.
To get the UDID, Apple writes:
Locating a Unique Device ID with iTunes 7.7 or later
Alternatively, you can also find out your device’s UDID using iTunes 7.7. or later. To do this, connect your device to your Mac and launch iTunes. 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.
This process works with iTunes for Windows the same way.