These are instructions for newer 7.1 versions.
To add a facebook connect button open inc/design.inc.php
Part 1. Removing the drop down menu. Look for the following at about line 477
'inputs' => array(
$aAuthTypes,
'nickname' => array(
'type' => 'text',
'name' => 'ID',
'caption' => _t('_NickName'),
),
Comment out $aAuthTypes,. Make it look like this.
This mod removes the auth type drop down list. This is optional.
'inputs' => array(
//$aAuthTypes,
'nickname' => array(
'type' => 'text',
'name' => 'ID',
'caption' => _t('_NickName'),
),
Part 2. Adding the button. Now near the end of the file at line 365 look for this.
$sFormCode = $oForm->getCode();
Make it look like this.
This mod adds the facebook connect button.
$sFormCode = '
<style>
.boxContent .form_advanced_wrapper {margin: 9px;}
</style>
<div style="text-align: center; margin-top: 8px;"><a href="modules/?r=facebook_connect/login_form"><img border="0" src="' . BX_DOL_URL_ROOT . 'templates/base/images/fbconnectbut.png"></a></div>' . $oForm->getCode();
NOTE: This has been setup for the boonex facebook connect. To use this code with a different module, change the href link.
Now you need a facebook connect button. Find one you like and put it in templates/base/images/ with the name fbconnectbut.png