Adding sites to Cloudboss is done in two ways.
The first one is to create a new VPS instance and add a new site to it from the "Add New VPS/Site" page.
This is covered in this video right here:
The second way is to add an add-on site to an existing VPS. This is done from the Live Sites page. You need to click on "Options" for the parent site and then on the "Add site" button.