All the documentation is present in the product detail.
There is nothing more required to just put a Google Key.

Here it is the copy of the detail description

Multi Sites Google Analytics is a free module for Joomla 1.5 and 1.6 that allows you add into your website the Google Analystics script code.

With this joomla module, you can assign different Google Analytics Tracker Identifier (UA-0000000-0) to different domain name. You can also assign a default identifier that will be used any domain name.
This Joomla Module is very suitable when you host multiple domain name to the same Joomla website.
This module is also works with Joomla Multi Sites.

Sample of configuration:

* www.domain1.com=UA-0000000-1
* www.domain2.com=UA-0000000-2
* =UA-0000000-3 <--- Default
* UA-0000000-3 <--- Default