Audiobook...help!

tried everything i can to get the nano to place files into the audiobook section of the nano. the files are all .m4a but theyre not going into the audiobook list. do i need to make an audiobook playlist?

can i manually change the .m4a to .m4b? i read somewhere on here thats what needs to be done. can it be done by hand? when i change it in the information to .m4b, a tiny padlock appears in the corner of the icon. does that matter?

ive tried to do everything ive seen in these pages, but nothing helps. you all are so smart, i figured id go to you with questions.