Here is a usefull snippet that I stole from qdap::url_dl to download files from my Dropbox to the working directory.
Argument x is the document name and d the document key.

dl_from_dropbox <- function(x, key) {
                        require(RCurl)
                        bin <- getBinaryURL(paste0("https://dl.dropboxusercontent.com/s/", key, "/", x),
                                            ssl.verifypeer = FALSE)
                        con <- file(x, open = "wb")
                        writeBin(bin, con)
                        close(con)
                        message(noquote(paste(x, "read into", getwd())))                        
                        }
# Example:
dl_from_dropbox("GViewer_Embeds.txt", "06fqlz6gswj80nj")
shell.exec("GViewer_Embeds.txt")

PS: Also see this R-package for interfacing with Dropbox