Modify > Font Families is new to Dreamweaver CS6. It doesn't exist in Dreamweaver CS5.
To edit the font list in earlier versions of Dreamweaver click the item at the bottom of the list of fonts. One place you can do this is in the CSS section of the Property inspector.
Select the fonts in the Available Fonts list, and click the double chevron to move them to the Chosen fonts list. Japanese example shown.
In Custom Font Stacks, there doesn't seem to be a way to reorder the fonts within a font stack, other than to remove all the fonts and start over. Am I missing something?? Is there a way to change the order of fonts listed in "Chosen fonts:" for a given stack? It's very frustrating to set up a stack, then realize that the order of fonts should be different, and have no way to change the order. Ugh!
No, there isn't a way to reorder the fonts in the font stack. It's less than ideal, but if you want to move a font up the order, you need to remove the fonts above it by clicking the right-facing double chevron until it's in the right position. You can only add new fonts to the end of the list.
Creating font stacks isn't something you do every day, so I don't regard it as a major inconvenience. However, it could certainly be improved. Why not file a feature request through the form at https://www.adobe.com/cfusion/mmform/index.cfm?name=wishform?