Install the builder is very easy, only thing you should do is to upload the right folder into your web server, then you can use it.
Upload both [ digith_template_builder ] and [ html ] folders into your webspace (make sure all required images stay available until your recipients will read the newsletter).
Now you can visit the template builder to creat your own page, the path will be [ http://youruploadpath/digith_template_builder/index.html ], make sure your web server support php file.