For those downloading and trying to unpack the Sigil.app-2.7.5-Mac-x86_64.txz , it seems Apple's Archive tool has a bug that will not let you unpack that file but all tests indicate that archive is fine and other tools can easily unpack it.
So you have 2 options:
A. Download the free "The Unarchiver.app" from the Apple Mac AppStore. It will unpack that .txz file just fine.
or
B. Use the built-in command line tar program to unpack it via Terminal.app as follows:
Code:
1. Create a folder on your Desktop called huh (or any name you like).
and drag and drop the downloaded Sigil*.txz file onto your Desktop
and into your huh folder
2. Open Terminal.app and enter the following commands into it
cd ~/Desktop/huh
tar -jxvf ./Sigil.app-2.7.5-Mac-x86_64.txz
Then once the extraction is complete, you should have a bright
shiny new Sigil.app folder sitting in that Desktop/huh folder.
To leave Terminal.app type in the following and hit enter:
exit
Then close Terminal.app to get it out of the way.
Open up the huh folder on your Desktop and drag and drop
the Sigil.app into your Applications folder. You can now trash the
huh folder you made on your Desktop.
I just tested this procedure on my old Mac laptop and it
worked like a charm.
If this issue continues in the future, we will file a bug report
with the Apple Archive utility but do not expect any bug
fixes from Apple any time soon. :(