It sounds like you are selecting the zip file you downloaded from SuCasa. In the zip from SuCasa, you will have several other zip files once you extract it. Make sure to select the SiteAssist Pro zip file when you import.
For example, let's take the "Investment Web Site" free template from SuCasa. When you unzip it, you get 5 more zips. For this layout "Investment_Layout.zip" is the one you want to import into SiteAssist Pro.