First, you would create you CSS Sculptor layout and make sure to save it as a preset. Next, open up the Preset Manager, then choose the SiteAssist Professional Layouts open. Click the New button. This will load up a 4 step wizard that will walk you through creating a new SiteAssist Professional layout. You will notice right on step 1, there is an option to use a CSS Sculptor preset. When you choose that option, you will be able to select the preset you save on the first step. After you complete the Add layout wizard, you're good to go and can use your new layout in SiteAssist.
You can also check out these steps in a document we already have available: siteassistpro_update_sculptor_mw_ht.pdf