Hi there. You don't need to download the MySQL Connector for .NET or change anything in Web.config to make mojoPortal work with MySQL. The procedure for configuring mojoPortal for MySQL is explained on the Using MySQL documentation page.
First, you should download the MySQL data layer file from the mojoPortal Codeplex Download page. When you place the files from that download into your web tree, as detailed in the ReadMe.txt file in the zip package, you will overwrite the MS SQL versions of them.
After that, you just need to put a MySQL connections string into your user.config.
I hope this helps,