Hi Ian,
You should be able to change the style of your SiteAssist menu quite easily. If you can see (in design view) the small javascript symbol next to the menu (see Number 1 in the attached image) then just click on that, and then you can choose to edit the menu from the first button that shows up in the properties inspector/panel (Number 2 in attached image).
If you can't see the little javascript symbol, then go to DreamWeaver Menu > Edit > Preferences > Invisible Elements > and make sure that the checkbox for 'Scripts' has been ticked.