I have an iframe:
<iframe src='myurl.php' frameborder='0' width='500' height='500' ></iframe>
First as dudzio said, use it simple like this:
<iframe src='myurl.php' frameborder='0' width='500' height='500'></iframe>
If you have another problems and you don't tell us corectly you can use: CSS, sample: position:absolute;top:X px; for iframe,
If your fieldset is not visible it may be z-index lower than another element on page.
We need the entire page to check why fieldset is below iframe... A lot of causes can be responsible for this. Please update you question!