Although WIAT-III Scoring Assistant is a standalone application, you can't run it directly because it's integrated through the PsychCorpCenter app, therefore you have to install that first and then the WIAT-III Scoring Assistant that acts as a module to the PsychCorpCenter. I've tested this and as you can see in the image below, you can launch it for there.
Another thing is that you have to activate the product after it has been installed. To do so, start PsychCorpCenter then click on the Home > Tools > Product Activations tab.
Here are also the download links for the programs: