<?php echo S2MEMBER_PAYPAL_API_SIGNATURE; ?>
This may output something like: Akdfw392-AnhvhsdfjLSJSKdtFuAmHxW3iD1N3GoZuZ8Vr2ii
(or whatever you have configured as your PayPal® API Signature)

---- s2member Shortcode Equivalent ----

[s2Get constant="S2MEMBER_PAYPAL_API_SIGNATURE" /]