If you have OSX Leopard 10.5 then converting from DMG to ISO is very easy, using Disk Utility.
Go to Applications>Utilities and double click Disk Utilty. When it opens you can either drag the DMG into Disk Utility
or, you can click on Convert
and select your image. Then in the Image Format at the bottom select 'DVD/CD Master'
Allow the file to be named .cdr, and when its finished you need to rename the image to .iso