For any new or existing page that you would like to show up on the top navigation bar, just add "@topnav" to the Tags field. To modify your existing pages, go to Manage > Pages. Click on it, and you should see a Tags field below the body.
I think Movable Type developers missed something when they created the Professional Website template. They hard-coded the tab links into the layout, meaning that when you create your own "Pages" in Movable Type, they don't show up as new tabs automatically. You still just get: Home | Blog | About | Contact To fix this, first you'll want to create a folder to keep your tab navigation pages separate from other pages. You might start creating pages that you don't want linked from the home page. ***Creating a new folder
(1) In UThink, go to Manage, then Folders.
(2) Click "Create Top Level Folder"
(3) Call it "Tab Navigation", then click Create.
(4) Now go to Manage, then Pages.
(5) Open up each page that you want linked from a tab and assign it your new folder, "Tab Navigation."
***Modifying the templates
(6) Now go to Design, then Templates
(7) Scroll down to the Navigation template module and click on it.
(8) Do you see the three
(9) In the same spot where you deleted, paste in this code:
<li class="<mt:If name="nav_on" eq="$this_page_id"> on</mt:If>"><a href="<$mt:PagePermalink$>"><$mt:PageTitle$></a></li>
Save and Re-build your blog site. That should do it!