To register a new client on the hosted landscape server follow the instructions linked in the grey box to the left of the screen when you have logged in to Landscape (right under where it tells you how many computers are registered in your account). The link text reads "following these instructions.".
Installing Landscape on your own network is a more involved task. For most use cases the hosted product is a good answer, but if you're in a situation where a local instance is appropriate the first step would be to start talking to Canonical. You can make contact here: http://www.ubuntu.com/business/services/contact
To learn more about Landscape Dedicated Server's architecture, see: