WordPress.org

1. Download the plugin file
2. Extract the archive contents to the wp-content/plugins folder i.e. wp-content/plugins/third-party-accounts-login
3. Login to WordPress Administration Center
4. Activate the plugin
5. Goto Settings -> Third Party Accounts Login and set your preferences. Currently, you can choose from three different views- Only text, small icons, X highlighted (large icons) + remaining small icons. You can hide the icons you do not want to display by double-clicking on them and drag them to order them.

6. Add the following line in comments.php: <?php third_party_accounts_login(); ?>

7. Make sure your OpenId textbox has an id="openid_identifier"

Sample code of comments.php will be something like:

<label >Please select one of these third-party accounts:</label><br/> 
<?php third_party_accounts_login(); ?>
<br/>
<label>Or enter your OpenId URL:</label><br/>
<input type="text" name="openid_identifier" id="openid_identifier"  class="textfield" tabindex="4" style="width:300px" />

For more details, please visit: http://anantgarg.com/wordpress-third-party-accounts-login-plugin