I had this problem with version 6.7.4 and resolved it by installing version 6.5.6.
My setup is Win 2008 R2 SP1 Data Center edition, SQL Server 2008 R2 with Business Intelligence Development Studio (VS2008). Very basic install.
When I was installing 6.7.4, i could not even see the MySQL provider as a choice. However, when i looked into the machine.config file, I saw references for MySQL role provider etc, but no entry was added in the .