I would suggest to get some support from http://www.aquoid.com/news/themes/suffusion/ the author of the theme.
The main reason why is that this will not be straight up answer from us here. Because Suffusion is wrapped up in WordPress hooks and few different stylesheets as well.
I did however paste the standard css menu for you:
http://pastebin.com/UPiCHPE2 pure css with dropdown option only for pages that you want to have dropdown for. This needs to be included in your stylesheet and in header.php where the code for your menu is.
This changes in your case because of the hooks and different type of skins this theme have.
Also take a look around the theme options, maybe there is an option for you to use standard menu, instead of single dropdown.