How to make selenium identify popup window opened by the server on client machine?

[Deepak G]

What steps will reproduce the problem?
1. Click on Submit button.
2. Server side form validation is initiated and when success, server
opens the popup window in client.
3. Unable to select popup window

What is the expected output? What do you see instead?
Selenium IDE command should be able to select(identify) the popup
window initiated by server on client machine and transfer control.

The popup window initiated by server is not selected(identified) by
selenium ide, returns error "window does not exists".

Selenium version:1.6
OS:Windows 7
Browser: Firefox
Browser version: 3.6
Web application.

[Deepak G]

<tr>
<td>open</td>
<td>/test/index.php</td> //URL of the component to be opened
<td></td>
</tr>
//Fill in mandatory fields in the form.
<tr>
<td>clickAndWait</td>
<td>name=generate</td> //Click form button to open pop up window
<td></td>
</tr>
//Before opening the pop up window system validates the form.
//After validation is passed server opens up pop up window
<tr>
<td>selectPopUp</td>
<td>name=generate</td> //pop up window opened by server
<td></td>
</tr>
<tr>
<td>click</td>
<td>id=pdf_0</td> //values to be selected in the pop up
<td></td>
</tr>
<tr>
<td>click</td>
<td>id=pdf_1</td> //values to be selected in the pop up
<td></td>
</tr>