Hi Marco,
That is not the right connetion string. The key mojoProjectMySQLConnectionString is for a separate project that never got implemented. You should be using the one named key=MySqlConnectionString
Also odbc connection is not supported, it uses ADO.NET connection provided by MySql.Data.dll which is the MySql connector for .NET. You downloaded the MySql data layer files separately and replaced the mojoPortal.Data.dll with the MySQL version right? And you put the MySql.Data.dll in the bin as well?
I would recommend restore the Web.config as it was fro the original download. Then open user.config.sample in a text editor and save it as user.config and set your connection string there. user.config can be used to override anything in the appSettings section of Web.config and makes it easier to keep your customizations when you upgrade. Be aware the .NET will detect changes to Web.config but not user.config so if you make chages to user.config you need to type a space in web.config and save it. This way .NET detects a change in Web.config and will reload all setings including those in user.config.
Is your web site configured as a root web site or as a virtual driector subfolder like localhost/mojoportal? If a virtual directory subfolder then make sure there is not another Web.config in the folder above it that interferes. Also make sure the folder above it is not configured for 1.1 .NET. From the error I can see that the folder that it is running in is 2.0 .NET but if its nested under a 1.1 app it could cause problems.
Hope it helps,
Joe