Trying to make bootable USB on OS X -- the disk you inserted was not readable by this computer ??

You've already got two working USB keys. You probably don't need to make another. Unless you're having an Ubuntu themed party, I suppose.

That's not an error related to the creation of the USB Boot Drive. That's an error you get after the dd operation completes successfully. OS X can't read the contents of a properly created Ubuntu boot drive. You now need to reboot and let reFind or the Mac's EFI loader find the drive.

If you're planning to install this copy of Ubuntu on your Mac, make sure you read, fully understand and then follow the guide for your specific year and model. If you don't, you'll have nothing but headaches.