Download the zip file (do NOT unzip it!).
Open the User Preference widget, install the add-on by choosing the downloaded zip file (e.g., split-toning-x-x.zip) from Install Add-on from File..., and then enable the add-on.
Go to Compositor and open the Add menu; you'll find a Split Toning option.
Once clicking the Split Toning option, a node will be added to the active node tree. The node can be linked to other nodes in a standard way.