accessing a form that is in an iframe
var ifr = document.getElementById( yourIframeId );
var ifrDoc = ifr.contentDocument || ifr.contentWindow.document;
var theForm = ifrDoc.getElementById( yourFormId );
Or you could have some code in the frame that sets a variable in parent to the form, but I wouldn't trust that method.
If your iframe has a name attribute, that can be used as a window name. If the frame name is "myframe":
myframe.document.getElementById("myform") // gives you the form element