In the original files of 3D Mentor, there was a mistake in the Max file for the Viola mode: HD_Vilola_01.max instead HD_Viola_01.max, so we wrote our catalog to use exactly this name.
Please, would you check what is the file name in your copy of HD Flowers 2 ?
Anyway, you also can use the models manually (without the catalog). Just merge the model in the scene, and assign to to Geometry->Custom Object.