To edit the menu using CSS menu writer, you will have to select the menu and in the properties inspector select edit menu. This may also help you with customizing your menus:
To import menu items or SiteAssist Pro layouts, you will have to go into the Insert->WebAssist->Preset Manager and select the correct option from the drop down menu. Then click Import and select the file that you want to import.
Make sure you take a look at the guides and tutorials on the support page, which will help you get started on using the products: