Install modman - https://github.com/colinmollenhour/modman
Run the following modman command:
$ modman clone https://github.com/Metrilo/metrilo-magento1-integration
Configure the plugin by going to "System" -> "Configuration" -> "Metrilo Analytics"
You can take the credentials from the Settings/Installation page of your Metrilo project.
Flush your magento cache.