How do I install Marketpress ecommerce plugin 2.8.5 from my desktop after downloading from WPMU?

Apologies for the cryptic question
regarding upgrading/installing the latest Marketpress plugin in a previous question. I trust this will make more sense.

I currently use Marketpress ecommerce plugin 2.6.4 on my wordpress site:
wwww.nextlevelexpeditions.com

Yesterday, after looking over the details of Marketpress ecommerce plugin
2.8.5 I thought I could benefit from the improvements of the latest version
of this plugin. So I bought it for $19 and downloaded to my desktop on my
MacBook Pro running OSX 10.8.3 and Safari. The download was a folder to my desktop but
no clear way/instructions to install. So, I assumed that I needed to zip
the folder in the same way that I have uploaded other plugins. Upon the
upload from my WP Admin page, the resulting message was "install failed". Obviously, I'm doing something wrong. I thought deactivating my current version of Market Press 2.6.4 my do the trick but to no avail. It would be great just to "upgrade" so I don't lose all of my existing data associated with my ecommerce settings.

Very best,
Kevin Noe