Setup key with:
sudo apt-key adv --keyserver pgp.mit.edu --recv-keys 5044912E
Setup repository with:
sudo sh -c 'echo "deb http://linux.dropbox.com/ubuntu/ natty main" >> /etc/apt/sources.list.d/dropbox.list'
Setup package with:
sudo apt-get update
sudo apt-get install nautilus-dropbox