Just to add on to what Jerry said, since dropbox will just sync anything that you add to it's folder, and as far as I know you can use any of the other methods in Photo Mechanic for moving or copying files to get them into Dropbox. So if you select your images and drag them a Dropbox synced folder in the Navigator, Dropbox will upload the files (Note: if you are on a Mac and Dropbox has forced you to move its folder to ~/Library, you won't be able to get to it from the Navigator, but you can add it to Photo Mechanic's Favorites and accomplish the same thing), and this would get around the fact that we can't upload to anything other than the Dropbox/Apps/Photo Mechanic Uploads folder.
Hope that helps.