To create Lesson Plan first you should prepare Class Timetable from Academics Class Timetable. Lesson Plan can be created teacher wise so once you created Class Timetable for the class so it will reflect in Manage Lesson Plan, select Teacher drop down.
Now, go to Lesson Plan Manage Lesson plan, select Teacher for which you want to add Lesson Plan (here selected teacher's time table will show in ascending in the below of the page ). Here at any period + sign button will show to Add Lesson Plan window for a selected teacher.