Make sure you're using at least WordPress 3.0.1 (you can find the version number in the lower right hand corner of your WordPress Dashboard).
For this plugin to be of any use, you must be using the network (aka MultiSite or MU) configuration. Not sure? Check the documentation.
Download the plugin and unzip the files to a familiar location, such as your Desktop or Documents folder.
Upload the "network-event-calendar" you created when unzipping the plugin to your 'wp-content/plugins' directory to your plugins directory. It will create a 'wp-content/plugins/network-event-calendar/' directory.
Log-in to your WordPress site as a SuperAdmin and then go to your main / root site Dashboard. Select "Plugins" and locate the entry for "Network Event Calendar." You may need to click "Inactive" to show the inactive plugins.
Click "Network Activate" to activate the plugin. If you do not see "Network Activate" as an option, you are either not running a Network/MultiSite installation, not logged in as SuperAdmin, or not viewing the root ( / ) site.
Requires: 3.0.1 or higher Compatible up to: 3.0.5 Last Updated: 2010-11-22 Active Installs: