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.
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.