Unibeast needs Install Mac OS X Mountain Lion.app inside the /Applications folder.
But what if I only have InstallESD.dmg ?
Below is what I did. In Terminal.app, do:
$ cd /Applications
$ mkdir -p "Install OS X Mountain Lion.app/Contents/SharedSupport"
$ cp InstallESD.dmg "Applications/Install OS X Mountain Lion.app/Contents/SharedSupport/"
Then open Unibeast, it should pick up InstallESD.dmg.
When Unibeast installs to my flash drive, The estimated time goes up to whopping 7hrs. I opened Disk Utility.app to see the used space of the flash drive, it was increasing. So I ignored the estimated time. At last the installation time did end up in about 5 minutes.