Sidebox - what file to add my info?

I know this has to be so easy..but I'm not a real programmer, I just play one on TV.
So I need simple instructions that lead me straight to the file where I change the text for the sidebox.
"
Replace this text with your HTML content.
You can include text, links, images, HTML markup and even PHP code"

I am adding code from the Constant Contact newsletter program to make a sign up box for this external newsletter program.

Maybe I'm in the right spot but Zen doesn't like this form code?

<!-- BEGIN: Constant Contact Stylish Email Newsletter Form --> <div align="center"> <div style="width:160; background-color: #ffffff;"> <form name="ccoptin" action="<http://visitor.constantcontact.com/d.jsp>" target="_blank" method="post" style="margin-bottom:3;"> <font style="font-weight: bold; font-family:'Comic Sans MS',Verdana,Helvetica,sans-serif; font-size:16px; color:#006699;">Sign up for our Email Newsletter</font> <input type="text" name="ea" size="20" value="" style="font-family:Verdana,Geneva,Arial,Helvetica,sans-serif; font-size:10px; border:1px solid #999999;"> <input type="submit" name="go" value="GO" class="submit" style="font-family:Verdana,Arial,Helvetica,sans-serif; font-size:10px;"> <input type="hidden" name="m" value="1101688662156"> <input type="hidden" name="p" value="oi"> </form> </div> </div> <!-- END: Constant Contact Stylish Email Newsletter Form -->

When I insert this code in the same place I changed the header text, I got an error message.
Any thoughts?

Thanks so much! Jennifer