Sidebar Appearance

have figured out how to add sub menus to a side bar, what I can’t work out is how to get them to display in the way I want them too. i.e. they physically display as a sub menu where as what I want to be able to do is hover over the side bar menu and for the sub menus to highlight (like it does on the top menu)
http://cpslgroup.co.uk/

Hope you can help