Need to create a landing page that does not show up in the menu.

Hi, Need to create a landing page (coming from PayPal), for a "Thank you" page and a "Sorry to see you leave" page.  I've tried make the pages private, but get a 404 error when I try to go to the page in another browser.  WP says you can hide a page from the menu system, For hiding pages from the navigation menu, follow these steps:

• Login to your WordPress dashboard.
• Navigate to “Appearance” and select “Menus”.
• Choose the menu you want to modify or create a new one.
• Locate the page you want to hide in the “Pages” panel on the left.
• Uncheck the box next to the page to remove it from the menu.
• Save the menu changes.

but it's no longer there after installing BizBoost theme.  Any suggestions?  Thank you.