Ahh, that’s a different case. I was downloading it from your link to my desktop.
Do you have access to your /plugins/ folder by ftp? If so, you can bypass your admin install problem by doing the same:
a) download the plugin zip file to your desktop
b) unzip it
c) upload the whole folder to your /wp-content/plugins folder on your server using ftp.
d) log in to your WordPress admin dashboard, go to Plugins, find the plugin (it should show up once in your Plugins area once its files are in the /plugins/ folder on your server)
e) activate the plugin from the Admin Dashboard > Plugins
That being said, I would also test to see if you can install any other plugins from the Admin, or if the problem rests solely with that plugin. If you can’t install ANY plugins from the admin dashboard, you may be seeing a symptom of a problem with your own files. Check it out and let us know!