Any chance that someone could give me the 101 on "navigating" webbrowser form objects?

Let me explain what I intend to do...I want to upload a file using http://mihd.net/ (and other similar sites like www.yousendit.com and such) but mihd.net uses an iframe in the main page, and I don't know how to navigate to that...also, I don't know how to do the same thing with www.yousendit.com (which is just a simple page with a load of boxes to be filled in...no iframes)

Possible ways I see of doing what I need to do:

1) Document.write putting the modified form back into the page and then somehow clicking the upload button...however I don't know how to set the focus to the iframe, as webbrowser.document.body.innerhtml will get the main page and not the iframe's HTML

2) Using elements (as in getelementbyid)...but I am HOPELESS at this and need help with understanding it :-)

3) Using cURL to handle the transmission...this is a long-winded way to do it, and I would rather not have to resort to having extra EXEs being included with the program I am writing (although it possibly will already have extra EXEs, I want as few as possible :-))...also, I would have to do a lot of messing about to find out what data to send using cURL for it to successfully upload the file

Help would be appreciated :-)