First, install Dropbox on your machine. When you install Dropbox, you indicate which folder Dropbox will keep synchronized for you. Make a subfolder under the dropbox root folder to store your Construct 2 backups. Make a note of the full path to the dropbox managed folder where you want your backups to be stored.
Use the instructions here to set your backup preferences, and specify the dropbox managed folder you identified above as one of the backup locations.