As an agent, you are automatically logged in to Live Assist when you log into the Dynamics 365 web client. (If you do not have login credentials, please request them from your Dynamics 365 administrator.) Once logged in to the web client, you can start using Live Assist, which appears in the right panel as illustrated below.
If the Agent panel is not loading ensure that the user has the Live Assist feature enabled in the Agent's Chat Settings: