With the built in menu the code below worked for me.
HTML Code:
li.rMenu-expand li a, div#menu1 ul.rMenu li.rMenu-expand li.rMenu-expand li a, div#menu1 ul.rMenu li.rMenu-expand li.rMenu-expand li.rMenu-expand li a, div#menu1 ul.rMenu li.rMenu-expand li.rMenu-expand li.rMenu-expand li.rMenu-expand li a, div#menu1 ul.rMenu li.rMenu-expand li.rMenu-expand li.rMenu-expand li.rMenu-expand li.rMenu-expand li a {
float: left;
}
You could also purchase AllWebMenus which has a plugin and extras to give you just about any menu you want. I just never wanted to spend the bucks for what I considered not much in return and there is a bit of a learning curve. The built in menus can be styled quite nicely.