Hello!
We have released patches that add the "Login with PayPal" feature to 4.6.0 - 4.6.2 versions (however, we recommend you to upgrade 4.6.2 to 4.6.3 instead of applying the patch).
You can download the patch in the "File area" section in the HelpDesk system at
https://secure.x-cart.com/ (look for "login-with-paypal-2014-03-12_{version}.tgz" file names). Instructions on applying the patch are in README file inside the package.
To communicate with PayPal servers we use the PHP code recommended by PayPal. Unfortunately, this adds extra requirements to your server:
1. You need this:
http://php.net/manual/en/curl.installation.php
2. The minimum PHP version is 5.3.2 (actually, any php version that has "sha512" in its phpinfo() result will work)
I've attached a script that you can use to check the new requirements. Place it in the X-Cart directory and open in your browser.
Thanks!