The problem is you are trying to run the WebStore project as the startup project and it can't run like that. You need to run the main web project mojoPortal.Web. There is a post build event that copies the needed files from webstore up to the main web. You can set break points etc in the code behind files of webstore, but you have to run it by running the main web app mojoPortal.Web. Create a page, add an instance of WebStore to the page like any other content feature. Create products, create offers, configure ecommerce settings.
In VS, right click the main mojoPortal.Web project and set it as the startup project, then hit play.
Hope it helps,