Since this is the first thread that comes up in google when you search for "MySQL- driver not loaded" error

update on this when it comes to mysql 5.7.10
the mysql 32 bit installation will install and leave you a x64 dll. this will not work with qt 32 bit qtmysql plugin

you do not need to build Qt mysql plugin since its already there when you install (tested with qt 5.5 : Qt\Qt5.5.0\5.5\[compiler]\plugins\sqldrivers)

what you need to do is install the mysql C connector ( not the C++ one) .
This will give you a libmysql.dll inside you connector lib path (C:\Program Files (x86)\MySQL\MySQL Connector C 6.1\lib)

use this dll with qsqlmysqld.dll that you already have inside the Qt folder.