you are using Safari as your browser, there is a pop-up setting on your iPad that needs to be to allow pop-up to open. You can follow the step-by-step instruction below:
1. Click on 'Settings' from the home screen of your device.
2. Tap the 'Safari' icon on the Settings page.
3. When the Safari preferences screen loads, scroll down to 'Security'.
4. Tap or slide the 'Block Pop-ups' switch to the 'Off' position.
5. Press the Home button to return to your home screen.
Pop-up Blocker should now be turned off and any pop-up window should be opened up.