1) extract the contents of the CMS zip file into a folder in your site, keep the folder structure exactly the same.
2) Power CMS already includes the administrative back end for adding content to the database. During set up, you create a admin user name and password that is used to access the back end.
power CMS is not configured to have multiple users, it will only accommodate one admin user. This is a feature that will be added to the next version.
you only need to create the site pages that will display the content you add to power cms, in other words the front end pages.