Using premium services usually is worth its money. Read here why!
Adding free accounts works the same way; this article explains how to add accounts of any type!
Go to Settings --> Account Manager --> Add --> Select desired host --> Click on "Buy premium"
You will be redirected to the correct page.
Do the same as above but this time, enter the login data and confirm by clicking on "Save".
Important: In some cases, you will need cookies instead of your password.
Click on the red text "Cookies" inside JDownloaders' GUI and a link to cookie login instructions will open!
Your account should be accepted within a few seconds and get displayed as a valid account in the account overview.