The easiest way to burn an ISO, the file you need to install Ubuntu from a CD, 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 CD/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.
Info collected from here