[Plugin: s2Member® Framework (Member Roles, Capabilities, Membership, PayPal Members)] Discount cod

Hello,
I have Wordpres 3.3.1 and S2 Member plugin.
I would like make discount option for new payment. New client can use discount code for 50% rebate!

Please tell me, how I made this!

Thanks
Matija

http://wordpress.org/extend/plugins/s2member/