Site sculptor generates the menu based on the pages you set it to create.
you can not generate the menu in the site design. the site design is used to setup the design elements that will be used for the site.
once you create the site design, you need to run the site assist wizard to create the site, and define the pages that will be created.
to create the site go to WebAssist -> Build Site -> New Site Sculptor Site.
From the site type list, select one of the site types, then click the define button to define the pages that will be created for your site. The tree under Main Pages -> Home pages section defines the items that will appear in your menu, and the pages that will be created.
from the Site Design list, you can select the Site Design You created.