The macOS/X package on Itch.io does not contain an exectuable when installed through the launcher. Itch.io automatically extracts the pkg(which is a modifed zip file), which produces three non-executable files (Bom, PackageInfo, and Payload). This can be fixed by downloading it directly from the browser (outside of the Itch.io software). After downloading, open the .pkg as a normal file. Then the software will install as normal.