Mount the .iso you downloaded, then restore the image inside it to disc/USB via Disk Utility.
I got it to work by doing a complete erase (not the quick erase) of the DVD+RW media in Disk Utility. This took about an hour.
Burning the IMG image inside the ISO didn't work, but burning the ISO to the erased media did. For those having trouble, remember to hold the C key down during boot.
Thanks!