How do I place super-fish or nice menus in Zen theme main menu?
Once you have installed Superfish, you must define a number of superfish blocks to be available at /admin/config/user-interface/superfish
Then go to the block admin screen and select one of the superfish blocks /admin/structure/block/manage/superfish/1/configure
Define the "Menu Parent" under superfish settings to "Main Menu"
Define the region you wish to have the menu placed in "Region settings"