@G.G is correct that read.xls does not support https. However, if you simply replace the https with http in the url you should be able to download the file.
Give this a try:
require(RCurl)
require(gdata)
url <- "http://dl.dropboxusercontent.com/u/27644144/NADAC%2020140101.xls"
test <- read.xls(url)