I agree, this is very hard to figure out. I too have several sites with contact forms that I had created in DW CS4 without Form Builder but wish to add the Captcha feature to them to avoid spam.
I have created a blank PHP page and added a Simple Contact Form I stripped away all the fields except for the Captcha stuff. I wound up with 132 lines of code.
I look at the Server Behaviors and there are none. There are however about 50 variables.
So, is there a really good tutorial somewhere to help digest what is needed to add Captcha to an EXISTING form that will make sense to even the noobiest of noobs?