Why do you need to open the InstallESD.dmg?
Just drag that file into your Disk Utility, and you can use the Restore tab to copy the image to a partition. 6GB should be great for the Mountain Lion installer.
You could also try using one of the apps that make the process easy. Lion Diskmaker works very nicely. I have several 8 GB flash drives, and have a couple dedicated for OS X installers. Lion Diskmaker (LDM) uses those, and erases them properly for best use with the Mountain Lion install. You can get that here: http://www.macupdate.com/app/mac/39701/lion-diskmaker
LDM will use a dedicated partition on a hard drive, or a USB flash key, or will also burn a DVD, whichever is what you want to do.
I suggest that burning a DVD is not worth your effort. Lion, and Mountain Lion installers are not optimized for booting from DVDs, and are really slow.
I think the best choice is an 8 GB flash key. My suggestion is to use a new flash drive just for the installer. You can find those everywhere for less than $10, and I have seen some for less than $6 ...
If that utility also doesn't like your Mountain Lion download - try restarting your Mac into Safe Boot mode (restart holding the Shift key), and try the same copy again.
If you still get the same error - then delete that download - it's likely just corrupted - and download again. You've already purchased the Mountain Lion install, so Apple doesn't charge you again.