Hi,
The version of mojoPortal in the Web App Gallery (the Web Platform Installer) is compiled for .NET 3.5, so when you upload your custom code that was compiled against a version of mojoPortal for .NET 4 it will not work. If you have .NET 4 hosting you should upload the mojoPortal package for .NET 4. You can download it from Codeplex and do manual upgrade. Or as Jamie says, really if you are working with the mojoPortal source code the best way is to produce your own deployment package built from the same code and including both mojoPortal and your custom features in the same deployment package.
There are differences between Web.config file for .NET 4 and .NET 3.5, this error makes me think your hosting is really .NET 3.5:
Config Error The configuration section 'system.web.extensions' cannot be read because it is missing a section declaration
because under .NET 4 that section does not need to b declared in Web.config because it is already declared in the machine.config file for .NET 4.
See breaking changes for .NET 4.
Typically if you declare the system.web.extensions section in Web.config under .NET 4 you get the opposite error about a duplicate section declaration whereas under .NET 3.5 if you leave it out you get the missing section that you posted because it does need to be declared under .NET 3.5.
Hope it helps,
Joe