I have installed the WP Smush plug in on my word press

Hello,
I have installed the WP Smush plug in on my word press site. Then I upgraded to the pro version. I am using a Mac and Safari and Firefox browers. I have tried numerous times to download the pro version, but when I get the file to download in zip file mode, upload it, and try to activate it- it says there is a fatal error. I'm not sure what to do to fix this. Can you please offer some advice.
Thanks,
Lisa