After you download Winnow and click on "winnow.dmg", MacOS will open a window like this:
Copy or drag winnow.app to your desktop or applications folder, BUT DO NOT REPLACE A PRIOR VERSION UNTIL YOU KNOW THE CURRENT ONE WORKS.
As I am not an identified Apple developer you will have to open the app twice.
STEP 1: Please either Ctrl click or Right click on winnow.app and pick "Open" from the dropdown menu.
STEP 2: You will see this dialog. Click OK. (THERE MIGHT BE A DELAY BEFORE THE DIALOG APPEARS)
STEP 3: Right click on the Winnow.app again, and select "Open" again.
STEP 4: This time the dialog will have an Open option. Click Open and Winnow should run.