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
Installation from sources
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.