Every loader system, whether it be first stage, second, third, twenty third, whatever, must be at one stage or another file system aware. The CD/DVD you are using, most likely has a file system on it. Your loader *must* parse this file system to find the file and its location. Even if you simply use blocks of data, your loader will eventually have to parse some kind of list to retrieve the correct block.
My opinion is not to start OS Development using a CD or DVD (.ISO image) as your starting media device. I would use a hard drive image which means that you can, with minor changes, make it work on a floppy, USB thumb drive, hard drive, etc., even a bootable CD once you get the idea of how they work.
As another note, you really need to give more information. Did you create your own boot loader for this ISO image? If you did, you should understand the file system the CD/DVD media uses.
Anyway, my opinion is to start with an easier form of media.
Hope this helps,
Ben
http://www.fysnet.net/osdesign_book_series.htm