this doesn't look right:
Unzip and upload the plugin to your plugins directory (wp-content/plugins/) when done wp-content/plugins/w3-total-cache/ should exist. If you have WordPress MU you will need to install this in wp-content/mu-plugins/w3-total-cache/.
wpmu does not look into subfolders, so placing the whole folder into mu-plugins doesn't work.
I suggest to put it into the plugins folder and activate sidewide.