When you activated Subscriptions, you can see Add new subscription option in Estatik plugin sections.
Click on Add new subscription and fill in the form that is opened.
Please note that Subscription period is period during which Agent is allowed to publish listings if he is subscribed to specific plan/subscription. For example if you set the form like is below, it means that agent can publish 5 listings per day during the week for $10. When week passes, he has to renew his plan and pay $5.
If you need to create Free plan, create it just entering 0 in Price field.