If you would like to automatically upload your Desktop, Documents and Downloads automatically to your Dropbox from your Mac, follow this nice little trick:
I will be using the “Documents” folder as an example:
Open Terminal (Spotlight/Terminal)
Change directory to your Dropbox folder (by default it is in the users home directory)
Create the symbolic link from your users Documents folder to Dropbox
ln -s ~/Documents
(Mine already exists)
Repeat for any other folders you want to back up automatically to Dropbox.