Dropbox is in the Repositories, so you can open Octopi and search for it. However it is a lot easier, for me anyway, to use pacman in a konsole session. The command is sudo pacman -S dropbox. You should out of habit precede that with sudo pacman -Syu to update the Repositories. After the install Dropbox will appear in the Application Menu under Internet. I set it up to execute on login, I don't recall if the install does that automatically so I always go to System Settings and verify. Enjoy