How can i add a block below the Main Menu?
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