You also have to make changes to the aircraft.cfg file. Just open it in a texteditor and add an entry about the checklist as in the example below.
Checklistfile: Cessna172SP_check.txt
Entry in aircraft.cfg:
kb_checklists=Cessna172SP_check
The entry has to be in the first part of the file, for example right after the 'texture=' entry. In most aircraft.cfg files the 'kb_checklists=' entry is already present even though it has not been given a value.
Hope this helps!
Happy flying!