It depends on what you mean when you say "Main Page".
When you launch WebHelp, there is something called the WebHelp start page. That is the page you point to in order to open the help with the full frameset. Often it is named Index.htm and the name of this page is configured here in the Single Source Layout properties:
Once the WebHelp frameset loads up, the first Topic is loaded. That is called the "Default Topic". And it is configured here in the Single Source Layout properties: