DNG is an adobe proprietary format so its obviously not converting properly. I have only used PS but I guess some of the principals are the same, if you have been editing in layers have you flattened the image before trying to save it? In PS you cant save as jpg until the image is flattened so this could be causing your issue. Also in PS you get the option to gradate the JPG quality which it sounds like elements does the same with the dual view.
From your comments about flicker which has a plugin, it isnt saying the image is an unrecognisable format just larger than the rules they have about image sizes, you could try re-sizing the image to a lower resolution for an upload only copy and see if flicker can then upload the dng file.
If that doesnt work then I might be able to help you out with a copy of PS cs5 for MAC...