Since there is no info about the software on Google or on the official website, I strongly recommend checking the built-in Help section because that's the only place where you might find the necessary instructions on how to add a SKU.
I've checked the official website, but they have other products now. You can contact them, although it might be possible that support for WashSoft to be discontinued.
You can get in touch with them through the following page: https://www.ints.com/contact