The way Apple has set up for developers to send their applications to users for testing before submitting to the App Store for general consumption is built around the idea of provisions and device IDs. Fortunately for you, dear user, you only need to worry about the device ID part (trust me, you don’t want to know).
Locating a Unique Device ID with iTunes 7.7 or later
You can 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.
Note that you do not need to highlight or select your device ID, simply Command+C when the ID is being displayed.

Now just email this long text (it is 40 characters of lowercase letters and numbers) to the developer who requested it, and await your preview version!
[Instructions originally from developer.apple.com]
Are there any security risks by sending this?
Nope. This is Apple’s officially sanctioned method for distributing samples.
We realize that redeeming iTunes promo codes can be a bit confusing, so we’ve written up these instructions to walk you through the process.
One important caveat to note is that you MUST have a US-based iTunes account. Fortunately, there’s a workaround if you don’t live in the US: it’s not obvious, but you can have multiple iTunes accounts for different countries. Simply follow the instructions below.
How to Redeem an iTunes App Store Promo Code
- Open up iTunes, and select the iTunes Store. Go to the Home Page by selecting the house icon near the top of the screen.
- If you do not already have a US-based iTunes account, first sign out of iTunes by clicking on your email in the upper-right corner and clicking the “Sign Out” button.
- On the right of the screen, there will be a “Quick Links” section. The first link will be “Redeem”. Click on the link, and input your promo code.
- If you already have a US-based iTunes account, then you’re done! Sync your device to install the application. If not, then you will be prompted to either sign in, or create a new account. You will need to create a new account now to finish redeeming the promo code.
- After the intro page you will be at the “iTunes Store Terms & Conditions” page. This is where you can change the country the new account is tied to, by click on the “If the billing address of your payment information is not in <your country>, click here”. Now you can select a country, such as “US”!
[adapted from: http://support.apple.com/kb/HT1917]
Enjoy your free stuff!