Google is your friend here.

Either you are building the plugin with the wrong dllimport / dllexport declaration or you are trying to use it in a program with the wrong declaration. You use dllexport when you are building the DLL and use dllimport when your are using the DLL in a program.

And in Microsoft world, you cannot mix debug and release mode code. If the DLL was built in release mode to use with Qt Designer, you cannot link with that same release mode DLL if you are building a debug mode program. You must build a debug mode DLL to link with your debug mode program.