Just login and activate the upload!
This application will upload your WhatsApp backup to your personal Dropbox account.
You can choose between an upload at 4:15 am and 1 pm, you can avoid uploads in mobile network, and you can decide whether you want all backup files to be stored or only the one that is most up to date. Of cause, last but not least, you can also perform a manual upload whenever you want to.
[This Application was neither created by "Whatsapp Inc." nor by "Dropbox Inc.", but is an independent product]