I don't know anything about cushyCMS. Have you looked at their documentation?
Also, it seems like they are a hosting company (the CMS is installed on their servers). Are you sure your site files are at Dreamhost?
The whole point of a CMS for the users is that one does not have to learn PHP/programming to use it. You may need to work with an expert to set it up and customize it and help you with security, but your day-to-day use of your site would be done through the CMS administration panel.
That's where you'd go to add something like a paypal button or new content. Do you have your log-in info?