Use fb:js-string to create an html body for the dialog, eg:
Show the dialog on page load (with the body 'searchDialogBod')
<script type="text/javascript">
var dialog = new Dialog().showMessage("Welcome to TopPoll", searchDialogBod, "Close");
</script>
Then define the body: (you can put any html in here)
<fb:js-string var='searchDialogBod'>
<center>
<fb:explanation><fb:message><font size="+2">Enter the name of a poll or any keyword</font></fb:message><form action="search.php" method="get"><input type="text" style="padding: 3px; font-size: 20px; width: 390px" name="s_q" id="s_q">
<input type="submit" value=" Search " class="dialognext">
</form></fb:explanation >
</center>
</fb:js-string>
Hope that helps