The ferry is there but sometimes the app don't select it for some reason. Then you can force add it to the plan. First make the plan with your destinations. Then zoom in outside the port of Sandefjord or Strömstad until you see a dashed line with the name Sandefjord-Strömstad. Then right-click on the line and click at Add ferry line. Then click on the blue planing button and the ferry will be added to the plan.