Chaos Cosmos Browser is included in the installations of V-Ray, Corona, Enscape, Chaos Envision and Chaos Vantage, and does not require any other specific actions. You can access the Cosmos asset and material library from your V-Ray, Corona, Enscape, Chaos Envision or Chaos Vantage UI menu.
See how to place 3D assets from Cosmos in your Vantage scene:
Note that when you open the Chaos Cosmos Browser for the first time, you have to log in with your Chaos credentials. To do so, click on the Sign-in button and enter your Chaos credentials. Then open the Chaos Cosmos Browser, download and import the 3D models, materials, and HDRIs you plan to use.