My first download of ML from Apple was corrupt and also gave errors when running Unibeast.
I only worked this out when I couldn't manually open the InstallESD.dmg file from the command line due to checksum errors.
Might be worth a shot at making sure your download is not corrupt before trying anything else.
Try running the following and see if the dmg file mounts.
# open /Applications/Install\ OS\ X\ Mountain\ Lion.app/Contents/SharedSupport/InstallESD.dmg