Using the Developer Notes
Sounds like you're headed in the right direction. Here are a few points that might help clarify some things regarding PowerCMS for you.
1. To ensure that they always work, place the first two lines of the Developer notes at the very top of your page - Above the <!DOCTYPE> reference.
2. The third line of code is different for each piece of content you've inserted into PowerCMS. You will paste this within the <body> tag of the page the content is to display.
For step-by-step instructions on adding these Developer Notes to your page...follow this How To powercms_creating_content.pdf.
The most common process for tying PowerCMS to an existing PHP site is to cut the content from your page and paste that content into a new content area in PowerCMS. Once you've inserted that content area, place the Developer Notes on the page the content is to display, inserting the third line in the exact location where you cut the code from. You should then find that it appears on your page exactly the same way as it did before...but you can now use PowerCMS to make updates and changes to that content.
What you insert into PowerCMS should be like any other content you would insert into the Design View of Dreamweaver. It should not have the <HTML> or <body> tags as these are already on the actual page.
Let us know if you continue to have issues. All the documentation and support for PowerCMS can be found on the PowerCMS Support page here powercms/.