Help with elgg Instructions

I am reading the instructions on how to install elgg in my root directory of my site. My provider is Dreamhost. I extracted all files within my root and created a MySQL database name and privilege. The instructions said to "open my elgg site" but what is it? Also when I opened one (www.mysitename.com/elgg) it said the statement below and I am trying to fix it:

Not Found

The requested URL /elgg was not found on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

Beginning Developers

Beginning Developers

This space is for newcomers, who wish to build a new plugin or to customize an existing one to their liking