If your browser's popup blocker settings are configured for maximum security, you will not be able to view any popup windows within Salesforce. This includes all popup windows, even those that provide necessary functionality such as the calendar popup for choosing a date on an activity, lookup dialog for selecting a record, the Help & Training window, and more.
To allow popup windows for Salesforce, add Salesforce as a trusted site within your browser's popup blocker settings. Some browser add-ons, like the Google toolbar, also have popup blocking. Consult your software documentation on those products for details on how to configure them to allow popup windows from Salesforce.
Add Trusted Sites in Internet Explorer (Step 1)

- Click the Tools button
- Click Internet Options
Add Trusted Sites in Internet Explorer (Step 2)

- Click the Security tab,
- Click security zone ‘Trusted sites’.
- Click Sites
Add Trusted Sites in Internet Explorer (Step 3)

- In the Add you will need to type *.salesforce.com
- Click Add.
Repeat
- In the Add you will need to type *.force.com
- Click Add.
- Click Close, and then click
Pop-up Blocker Settings in Firefox (Step 1)

- At the top of the Firefox window click on Tools
- Click Options On the menu bar
Pop-up Blocker Settings in Firefox (Step 2)

To access the pop-up blocker settings:
- Select the Content panel.
- Select Exceptions: This is a list of sites that you want to allow to display pop-ups.
Pop-up Blocker Settings in Firefox (Step 3)

- In the Address of web site: type in www.salesforce.com
- Click Allow to add to the exceptions list.
Repeat
- In the Address of web site: type in www.force.com
- Click Allow to add to the exceptions list.
- Click Close