If you got the mssql 2-2-4-0-preview download you need also the mysqldatalayer download and replace the mojoPortal.Data.dll in the /bin folder and add the MySql.Data.dll from the mysqldatalayer download. The can't find file message sounds like you didn't do that part.
If you've already done that, then I would try making the add assembly setting match the one its asking for. If that doesn't work, try removing the add assembly setting.
If that doesn't work I would ask your host to install the newer MySql connector in the GAC.
Hope it helps,
Joe