GLM won't/can't(?) automagically cast a mat4
to GLfloat*
so you have to help it along a bit.
Try this:
#include <glm/gtc/type_ptr.hpp>
glMultMatrixf( glm::value_ptr( glm::mat4_cast(orientation) ) );
This might also work:
glMultMatrixf( &glm::mat4_cast(orientation)[0][0] );