Step 3 of the checkout process, down the bottom of the screen there's a checkbox option for 'redeem a coupon'. Check the box, click 'continue checkout', and you'll be able to enter the code.

Coupons ain't dead... they're still floating about. Check the Performance Bike.com thread in the Affiliates Link section for current known coupon codes. Use the link to Performance Bike website in there for online orders and Bikeforums.net gets a slice of the action.