You can install Dropbox in a number of ways. The simplest process is to use the Ubuntu Software Center.
Open Ubuntu Software Center and do a search on Dropbox:
Click on Dropbox and follow the prompts to install the Dropbox.
Note*: In the list above there is an option for Caja Dropbox. We suggest that you download and install that version if you are running Ubuntu Mate.
To install darktable via terminal. Open terminal via menu or via keys CLT+ALT+T.
sudo apt install dropbox