The first thing you need to do is download the latest version of the plugin. Unless you really know what you're doing, choose the binary rather than the source.
This plugin installs quite a few files, so it's best to let HTML-Kit handle the install for you. In HTML-Kit, go to Tools | Install | Install Plugin... and select the file you just downloaded (you probably need to change folders). You will then have to restart HTML-Kit.
By default, hkTreeView puts an icon onto the Tools page of the actions bar. You can right-click on the icon and select Customise... to change this. To start the plugin, make sure you have an HTML or XML file as your current document and then click on the hkTreeView icon.
Note: To avoid confusion (or possibly to create it), the plugin is called 'cm Tree View' by HTML-Kit. It has become conventional to prefix the name of plugins with the author's initials in case plugins start overwriting each other. Just so that you know :)
The interface should be fairly intuitive. If you can't figure out what a button does just place your mouse over it and read the status bar. Alternatively, just click on things at random and watch the fireworks.
Giving The Author All Your Money
The final stage in becoming a truly professional user of the plugin is to write a big fat cheque to the author. Send me an email for my postal address. Note that Chami deserves the money far more than I do, so don't forget to make him a millionaire as well.