Hi Pat,
Sorry, but you can't. It is one of the top items in 'how to improve on VSD' section of this forum. You have to use some outside tool to make the drop down menu first, like cayucustom already mentioned, and then add the code to VSD with the HTML box. Coffeecup has a free tool for you to do this: The DHTML menu maker.
The problem with all these tools is: they are super clumsy to use in combination with VSD. After you've made the menu, you
have to link each individual page to one of the menu items, which means you'll have to design your VSD site page by page, without any interconnection.... First the home page with the drop-down menu and some general information, then each and every page separately, upload these pages to your server, and then change the drop-down menu on the homepage to link to all these pages.... If that isn't inconvenient....
The best thing you can do is to forget this and wait till (if ever) a drop-down menu maker is added to VSD. In the meantime, if you really want drop-down menus, try to learn the HTML editor instead.
I made a VSD example of a combination of horizontal and vertical menu items some time ago. Maybe this might be an inspiration for you to use VSD without drop-down menus, but with some clever page switching, giving a lot of possibilities. See it in action here:
http://johnvanhulst.com/probike/ You can download the VSD design here:
http://www.johnvanhulst.com/probike/probike.vnu
Success, John
John van Hulst