If you are extending your YJSG based theme this is the place to start.
Besides 5 menu types YJSG comes with additional 3 Bootstrap menus. To activate Bootstrap menus, go to your Menu Module manager, tab name Yjsg Menu Options and from parameter Select menu type, select either Bootstrap nav pills, Bootstrap navbar or Bootstrap navbar inverse.
Mega menu is not functional when you use Bootstrap menu styles and Bootstrap menu type will show maximum of 3 menu levels. Note that Bootstrap menu requires either Bootstrap2 or Bootstrap3 to be activated in template manager.
Go to your template manager tab name Top menu and set Default Menu Style to Menu module position :
Go to your Menu module manager and assign it to topmenupoz
module position:
navbar navbar-inverse
class name can also be used if you wish to have dark navbar menu style;