If you go (in the "gears" menu) to "Appointment Management" => "Exam Flow Management" and then click "Edit Clinic Default" you'll see a button near the top that says "Enable Rich Text Treatments":
If you click on that and then click "Save", then the next time you check-in an appointment and click "Add Treatment" under an Assessment, you'll see a new checkbox on the right-hand end of the Treatment line allowing you to switch to rich text.