Select “Articulate interests…” tasks and create a new one, This is exactly what is shown on the screencast above, unless I don;t understand what do you mean by “main task”.
I understand that IF I want new task as a sibling task, I go to the selected task and create a subtask.
I probably was not completely clear. New task is created on the same level with the selected task. If your selected task is a top-level task, you create a new top-level task. If your selected task is a child of some summary task, you create the new child of the same summary task.
You may also just unindent the new task using Shift+Tab or unindent buttons above the task table.