You don't have to compile it for Qt: The mpusbapi.dll is compiler independent! Also one sentence later:
C++ source code for MPUSBAPI is provided.

The _mpusbapi.h file must be included in every file that uses
the MPUSBAPI functions.

The mpusbapi.dll file must be in the same directory as the
executable file.
What do you need more? Have a look at LIBS in the qmake documentation. If you need the lib then you have to compile the provided sources with your compiler. Build instruction can surly be found inside the project.