To use Citavi 6, you need to log into Citavi . On the welcome screen of Citavi 6, click on the login icon on the right: 
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This window is shown from version Citavi 6.10, older versions of Citavi fo Windows show directly the login page.
A click on Log in opens your default web browser with the login window.
You can now log in with the email address of your existing Citavi account. As an alternative, you can also enter with your Google, Facebook oder Microsoft account or use the access provided by your school ("Shibboleth").
Once you have entered the information, your browser will ask you to pass the login to your Citavi desktop application. To do so, click first on Choose application, and then on Open Link. The text shown on the button might differ slightly depending on your web browser.
Your Citavi application will now ask you, if you want to remain logged in on the computer your are using at the moment. Choose one of the two options and confirm hitting Next:
You are now successfully logged in.
If the login was not successful, check the following articles depending on your Citavi version: