Download the tar.gz file from the same download page.
The install scripts assume a fresh install of the OS or that you aren't using cacti, mysql, or postgresql. The scripts overwrite a lot of the configs. Examine them before you run them.
I'm going to attempt to install on CentOS first, and then Ubuntu. I'm planning on posting the results and any fixes workarounds on my blog here : http://weblog.sharova.net