So you create a new menu with your items first.
You then save your new menu.
With your menu created and saved, you then use the dropdown over on the left to assign the menu to the Primary position and save that.
Are these the steps you completed, and still your custom menu does not show up?
If the above is all true, have you made any modifications through your child theme aside from style.css? Are there any other files in your child theme?
Are these the steps you completed, and still your custom menu does not show up?
That is correct.
Are there any other files in your child theme?
No but…
At one point I had trouble with a reply box showing up on every page. I fixed that by commenting out ‘$comment = get_comment($comment_id);’ in file wp-comments-post.php in the twentyeleven theme, not in my child.
At this time that change does not appear in twentyeleven and the reply box has not reappeared. I don’t know why.