Sorry for not answering this post earler., been swamped with my own Moodle projects and neglected the Lesson forum Will do this from memory.
1a) As a teacher, click on the lesson link. You will/should enter it and see the tabs at the top, usually at the "Preview" tab. Look up in the top right corner of this page and you should see a button "Update the Lesson" or something like that. That is the link to Lesson settings.
1b) As a teacher turn on editing in the course. Look for the hand icon next to the lesson. That is the link to Lesson settings.
2) In Lesson settings (Updating Lesson), can you tell I think settings really says it all) Now you should see the setting for custom scoring (yes or no).
3) Remember in a multiple choice question set to multiple answer, all the answers must be correct. See this in Moodle Docs Lesson_question_types#With_multianswer_box_checked
Hope this helps, Chris