There are two places where you can add notes.
Family Notes Screen
On the Family Notes page click on the paper icon with the plus in it to add a Note.
Family Member Channels Screen
When on a Channel Details Screen of a Family Member you can add a note by clicking the "+ Add Notes" link
Notes Fields
The following fields are available on the Notes add screen
Cubby: The Family Member this Note pertains to
Channel: The Channel this Note pertains to
Title: What you want to call the Note, this is what shows up in the list
Link URL: If you want to add a URL to the note (optional)
Notes: Text that describes the note or the text that the note is about (optional)
Attachment: Upload a file (future)