The easiest way to burn an ISO, the file you need to install Ubuntu from a DVD, is by using Disk Utility. If you’re using an older version of Mac OS X, you might not have the Disk Utility application. In this case, use Disk Copy.
Launch 'Disk Utility' (Applications → Utilities → Disk Utility).
Insert your blank DVD.
Drag and drop your .iso file to the left pane in Disk Utility. Now both the blank disc and the .iso should be listed.
Select the .iso file, and click on the 'Burn' button in the toolbar.
Ensure that the 'Verify burned data' checkbox is ticked (you may need to click on the disclosure triangle to see the checkbox).
Click 'Burn'. The data will be burned and verified.
And if you get stuck… Ask Ubuntu