The menu tabs at the top appear when you create new pages, not posts.
The links you have on your sidebar are external links to html files, not pages created within WordPress. Are those the tabs you want to create in the menu?
The options I can think of are two:
1 – create new pages in WordPress and paste the content of the html files there
2 – hard code the menu tabs in the header, as below.
In your theme folder, open header.php. You already have “Home” and “About”, then add your html links, like this:
<ul id="nav">
<li class="page_item"><a href="https://www.nationaleconomy.net/blog/" title="Home">Home</a></li>
<li class="page_item page-item-2"><a href="https://www.nationaleconomy.net/blog/?page_id=2" title="About">About</a></li>
<li class="page_item page-item-3"><a href="https://www.nationaleconomy.net/order.html" title="Order">Order</a></li>
<li class="page_item page-item-4"><a href="https://www.nationaleconomy.net/soddy.html" title="Frederick Soddy">F. Soddy</a></li>
<li class="page_item page-item-5"><a href="https://www.nationaleconomy.net/chapters.html" title="Sample Chapters">Sample Chapters</a></li>
</ul>
That will work.
Also, add something to the “about” page, then publish it, and the link will change from /blog/?page_id=2
to /blog/about
Hope this helps!