Hiya,
Check that you have Scripts ticked in 'Invisible Elements'.
Go to DreamWeaver menu > Edit > Preferences > Invisible Elements > check that the 'scripts' checkbox is ticked.
In the page with your menu in it, in design view, you should now be able to click on the script icon to select it, and the CSS Menu Writer option buttons should be available from the properties inspector/panel.
On an HTML page it seems you need the javascript icon to click on. On PHP pages, clicking on the menu seems to be enough.