This article explains how to install and open WireframeSketcher on macOS 10.15 Catalina. Starting with macOS Catalina, Apple requires that Mac software distributed outside the Mac App Store must be notarized by Apple in order to run.
At this moment WireframeSketcher is not a notarized yet, although it is digitally signed with a Developer ID certificate. This results in the following warning being displayed when WireframeSketcher is opened on macOS Catalina:
To dismiss this warning, click the Show in Finder button, and then, in Finder, right-click on WireframeSketcher app. Choose Open from the context menu, and then click Open button in the warning dialog:
WireframeSketcher will launch. You need to do these steps once, and then WireframeSketcher will launch normally the next time.