First and foremost, ensure that you have discussed Client Discovery with your sales professional if you have not yet. It is an add-on feature with value to those who are wanting to improve their discovery process.
Secondly, once Client Discovery is triggered on it will show Discoveries and Clients over in the left sidebar.
When you first begin you will need to designate Client Companies & Contacts find out more about this here.
When you first begin you will need to designate Client Contacts find out more about this here.
Additionally, you can add a client in the midst of the creation of the Client Discovery, see how to do that here.