Place the DMG you would like to convert on your desktop then open a new Finder window by clicking its icon in the dock.
Click to select Applications from the list of Places on the left.
Double click to open the Utilities folder from list of Applications.
Double click to launch Terminal from the list of Utilities.
Enter the following command into the Terminal window.
hdiutil convert ~/Desktop/'filename.dmg' -format UDTO -o ~/Desktop/'filename.iso'
NOTE*: Remember to replace 'filename' with the name of your DMG image.
You will now notice a new file appear on your desktop with a .iso.cdr extension. Select the file and hit the Enter key to rename it removing the .cdr extension.
You may be asked to confirm this. Click to select Use .iso.
You can now copy this to any Windows or Linux computer for burning.