Since the right "workspace" section of the Manager is already a frame, you don’t need to do that. Take a look at one of the .php files in manager/actions. When creating a new module, all you need to do is create a page as if it were stand-alone. Usually I just have the basic layout of the frame, then inline php to include the rest of the php code for actually running the module.
http://www.sottwell.com/create-module.html
With DADA Mail, all I did was provide a link to pull the DADA script’s output into the frame.
I had forgotten, though, that DADA doesn’t run on Solaris, so I had to switch that client to the
PHPList script. I have it running as a separate script from MODx, which seems to have made the client happy since they do frequent newsletters but rarely modify their site, and the secretary does the newsletter anyway, so I haven’t tried to integrate it. Another difference is that this client does not want auto-registration; they have a contact form and anyone requesting to join the list is contacted and manually added to the list. They do not want a web login for their site. So my choice probably shouldn’t influence yours all that much.