How can i add a block below the Main Menu? I asked for a second horizontal menu here. But i couldn't manage to success so i'm changing my question a bit.

I want to add a block below the "Main Menu". I will put a table into that new block, which will be looked like a horizontal menu.

In my default case "Main Menu block" is disabled. But i can see the Main Menu horizontally. I ccan't understand how it can be seen :) When i enable "Main Menu block" from "admin/structure/block" that block becomes visible where i set its position, but it sorts Main Menu Items vertically. Also horizontal Main Menu still appears. (That case is also occurs for "User Menu". Its block is disabled. When i enable its block it comes with a vertical menu)

What's the position of the "below of the horizontal Main Menu" in Bartik theme? I created new block and put it to highlighted, help, featured regions. Help and highlighted regions doesn't cover all horizontal part. Featured tooks three times more place than the block.

So i couldn't find a way to put my new block just below the Main Menu..
I would be happy if you can help. Thank you.
The main menu that you can see in the theme is not in a block.  It's printed at a certain spot within your page.tpl.php page in the bartik theme.  Check it out (make a copy first) and mess about to see the results.
If you want to add a new block to a region that doesn't already have a block check out this post http://drupal.org/node/1088718 as it details how to do so.
Your other question had to do with why the menu renders vertically instead of horizontally like the main menu, it's all due to the css styling.  Chances are that the menu is in an unordered list &lt;ul&gt;&lt;li&gt;&lt;/li&gt;&lt;/ul&gt; and that the list items (menu links) have been set to display:inline or float:left.
I hope I was of some help