You currently have zero published pages on your site:
The three pages you have listed under Pages -> All Pages are privately published. Privately published pages are only visible to the author of the pages and are not able to be added to a Custom Menu. Only pages that are publicly published can be added to a menu.
PLEASE NOTE: Your site's privacy settings are already set to Private - which means that only people you invite to view your blog will have access. Publicly publishing a post/page will only make the post/page visible to people who have access to your site.
If you change the privacy setting on each page to Public, they will be able to be added to your Custom Menu.
I hope that clarifies things! Let me know if you have any questions.