Hi chuna and welcome to cdfreaks.
I'm not sure which application you're using, but I believe the navigation for what you ask is the same. On the opening screen, note the "source" and "target" drop down menus (top of the screen). To the right of each, you'll see two icons...the icons just to the right are the dvd folders you're already using. The icons on the far right (image of a disc with a page overlay) are the .ISO icons.
So to rip to an .ISO image, simply click the .ISO icon to the far right of the "source" drop down and choose a location.
Similarly, to burn an .ISO file, click on the .ISO icon to the far right of the "target" drop down and brose to the correct file.
Hope this helps.