Locate the Services box in the client area and click on it.
Usually you should find a list of different services, which are connected to your account. Locate the Plesk service and click on the table row. (Hint: Avoid clicking on the domain name itself, this would lead you to the URL and not to Plesk).
Locate the "Login to Plesk Control Panel" button and click on it.
A new tab will open up and you will be logged in via Single Sign-On directly in the Plesk Control Panel as the subscription/s owner.