Trying to figure out just exactly how to find your Mac UDID so you can get in on an ad-hoc Mac App Store beta? Just like with iPhone and iPad, your Mac has a UDID (technically a Hardware UUID) that developers can use to send you app builds before they become publicly available. Luckily, it's easy to locate once you know where to look.
- Click on the Apple logo at the top left of your Mac menu bar
- Click on About This Mac
- Click on More Info...
- Click on System Report
- Make sure Hardware is selected at the top left
- The very last entry in the list on the right will be your Hardware UUID
Copy and paste it and mail it off to the developer, then wait for your ad-hoc app to arrive and start beta testing!
Of course, since the Mac App Store isn't the only way to distribute software on the Mac, developers can also choose to simply send you a non Mac App Store -- i.e. regular-style apps. If they're developing specifically and only for the Mac App Store, however, this might be the way they choose to go. If it is, you're all set!