Each time you go to the doctor, post a new Activity item for that appointment. Choose the category "Therapy", enter the date and time of the appointment, and in the "Type" field, enter the name of the doctor. If you've already entered this doctor's name in a previous Activity you posted, Birdhouse will remember and suggest it to you when you post it again. Click "Save this Activity".
Using the Search field on the right of the Filter bar (you can also search in the iPhone and Android apps at the top of the Activity screen), you'll be able to find all Activities you've posted for whichever doctor you're searching for, and the notes for each appointment will be right there, too.