Unable to switch control to a prompt with multiple fields in selenium - selenium

Question

I am trying to select an element in the prompt, but I am unable to switch control to the prompt.
I've tried the below code:
Alert a = driver.switchTo().alert();
WebElement e = wait.until(Expected Conditions.visibilityofElementLocated(By.xpath("locator"));
Adding HTML code
<table role="dialog" aria-labelledby="ariaLabelledBy_promptsDlg" oncontextmenu="_CWprompts.eventCancelBubble(event);return false" border="0" cellspacing="0" cellpadding="0" id="promptsDlg" style="display: block; padding: 0px; visibility: visible; position: absolute; top: 119px; left: 368px; width: 800px; height: 400px; z-index: 1005;">
<tbody>
<tr>
<td style="width:800px;height:400px;" class="dialogbox" id="td_dialog_promptsDlg" onresize="_CWprompts.DialogBoxWidget_resizeIframeCB('promptsDlg',this)" valign="top">
<table class="dlgBox2" width="100%" border="0" cellspacing="0" cellpadding="0">
<tbody>
<tr>
<td height="0" valign="top">
<table style="height:26" class="dlgTitle" width="100%" border="0" cellspacing="0" cellpadding="0">
<tbody>
<tr valign="top" style="height:26px">
<td onselectstart="return false" ondragstart="return false" onmousedown="_CWprompts.DialogBoxWidget_down(event,'promptsDlg',this,false);return false;" style="cursor:move;padding-left:10px;" width="100%" valign="middle" align="left">
<nobr>
<span id="ariaLabelledBy_promptsDlg" class="titlezone">Prompts</span>
</nobr>
</td>
<td class="dlgCloseArea" align="left" valign="middle">
<table border="0" cellspacing="0" cellpadding="0">
<tbody>
<tr style="height:26px">
<td valign="middle" align="left" style="padding-right:4px;">
<div id="helpIcon_promptsDlg" "="" title="Help Contents" class="imo" style="width:16px;height:16px;background-image:url('images/main/galleries/icon16x16gallery1b.png');background-position:0px -480px;"></div>
</td>
<td style="padding-right:10px">
<div id="dialogClose_promptsDlg" class="dlgCloseBtn" title="Close window"></div>
</td>
</tr>
</tbody>
</table>
</td>
</tr>
</tbody>
</table>
</td>
</tr>
I am getting no such alert exception. Can someone please help me out with this?
Thanks in advance.