Thunar Dropbox is a plugin for thunar that adds context-menu items from dropbox. This plugin does not come with dropbox itself, you will need to install that separately.
Depending on your distribution you might have to install libthunarx-2-dev.
sudo apt-get install libthunarx-2-dev
Installation from source
python2 ./waf configure
python2 ./waf build
sudo python2 ./waf install
Note: the provided waf does not work correctly with python3, so make sure you use python2 to run these commands.