Topaz Fusion Express is a plug-in that imports Topaz Photoshop plug-ins into Aperture, Lightoom and iPhoto, providing users with the same image editing capabilities and user experiences that Photoshop users have with Topaz Photoshop plug-in products.
So basically, Fusion Express is a plug-in for a plug-in. When you install Fusion it installs the Topaz programs: Adjust, DeNoise, Simplify, Clean, Detail and DeJPEG.
Fusion itself it not a program that you can run and use. Rather, it serves as a portal that will allow you to access your Topaz filters in Aperture, Lightroom and iPhoto, and use them just as you would in Photoshop.