Once you have purchased a reward from the rewards store, that reward will appear under "My Rewards" from the “more” tab.
When you're ready to check out, tap the reward in “My Rewards” to open it. It will display a unique 6-digit code. Show this code to a member of the staff. The reward will automatically be applied and taken off your phone. Note that when you scan your receipt for points, you'll earn points on the new total after the discount has been taken off.