First you should install Facebook connect plugin by following the instructions here

1. Find the Facebook Connect plugin - move mouse cursor over it, and click the Settings button.

2. You will be directed to the Facebook connect management page.

3. Follow the steps described on this page:

• Click Apps > Add a New App in the top menu.

• Choose the Website option.

• Click the Skip and create App ID at the top right corner.

Enter your application Display Name and Contact Email.

• Select a category and click Create App ID.

You will be directed to the application Dashboard page.

• Click Settings tab in the Sidebar menu.

• Enter your site domain name (e.g.:www.yoursitedomain.com) and Namespace.

• Click Save changes.

• Click Add Platform » Select Website

• Enter Site URL: http://www.yoursitedomain.com/ and click the Save Changes button.

• To publish your app click App Review tab in the Sidebar menu.

• Set Make mypageappid public? from No to Yes

Click Confirm in the Make app public? window.

• To upload your site logo and add additional information about your website, click Settings in the Sidebar menu.

4.Go to Settings tab, copy the Application ID and App secret

Add them into Admin Area > Plugins > Installed plugins > Facebook Connect plugin.

5. Click the Save button.