How to create on mouse over dropdown menu in drupal 7?
You can use nice menu or super fish menu
nice menu will create two blocks called nice menu 1 and nice menu 2 you have to choose on of them as a block and in configuration you will choose a menu to display

also superfish module will create 4 blocks (it is a dynamic number form config menu) and give you more options to style your menu also option to choose which menu you want to display