mojoPortal knows which database to use because there is always only 1 version of the data access dll files. In other words we have different versions of mojoPortal.Data.dll and mojoPortal.Features.Data.dll and WebStore.Data.dll for each supported platform. Rleases packages are created for each db platform and include the correct dlls for that platform.
Your error message refers to "MySql.Data.MySqlClient" which clearly means you have the MySql versions of the data dlls. In medium trust, if you have a choice between MS SQL and MySql, I recomend you go with MS SQL because you will have less trouble that way.
So my recommendation is download the mojoportal package for MS SQL (mojoportal-2-2-7-3-mssqlreleasefiles.zip) and install that and do not use the MySql data layer.
Hope it helps,
Joe