How to add Main Menu to Menu Bar area?
Menus are not blocks, and as such cannot be placed in regions. Some themes don't render menus on their own, expecting you to place them in a region you prefer. Sounds counter-intuitive, but it can be solved.

To make menu a block, you can use Menu block module.


  It provides configurable blocks of menu trees starting with any level of any menu. And more!


Alternatively, you could use Nice Menus module:


  Nice Menus creates blocks that may be associated with any existing site menu which can be placed wherever normal blocks can be placed in a theme


Blocks the two modules are generating are different, but both can do the job. Test which one is better handled by your theme, and if both are good, use the one you simply prefer.