For those with a Mac that are having problems this is probably why.
You insert the blank DVD into the drive, you open the DVD, drag the iso into it and then burn. This method will not work.
For Mac users the correct way is this:
Go to the application folder open Utilities
From there open Disk Utility
Go to File>Open Disk Image> Select the unzipped iso file.
Drag the iso file and drop it into the left side column.
On the left side column of Disk Utility you will see the iso file, click it and select burn. It should work now.
Here's a youtube video to make it easier: