Adding post type posts to WP Nav Menus
If you want to add custom post type posts, you first need to make sure each desired post type is getting the necessary parameter. Once it is, you should see a section for it on the left side of the menu builder screen.
Adding the support.
When editing your post type, locate the "Show in Nav Menus" option and make sure the associated dropdown is set to "true". It is set to be true by default.
Checking the menu builder screen.
Once you have ensured all your desired post types have the support, visit "/wp-admin/nav-menus.php" and confirm seeing each post type along the left. Each will get their own collapsible section.
Debugging missing listed post types.
If you are not seeing some or all of them listed, click on the "screen options" tab at the top right corner of the screen and double check that each has the associated checkbox checked. This is how WordPress toggles them in the builder screen, and they may not be toggled right away.