Once you log in, you will be able to see the "+Add Domain" at the top.
After clicking on it, you will be presented with the ability to add a domain.
Once you have typed in your domain all you need to do is to click on "Add Domains" and you are good to go.You will then be redirected to the DMARC record generator, where you can generate and publish your DMARC record.
Alternatively, you can set up your account by going through the setup wizard.