Aaron is correct that the VS designer is fairly worthless to begin with, even more so in mojoPortal because of the way we support multiple sites and multiple skins and the way the location of skin files are determined differently at runtime than at design time in VS. I never use the designer in VS myself.
That said, I don't get any errors in design view on Default.aspx. Your screen shot looks like it shows your custom pages from your custom feature, and on the right side it shows the mojoPortal.Web project as unavailable which would account for the errors.
Best,
Joe