Promo codes can only be redeemed on a subscription purchased through the Stitcher Website directly and cannot be used on app store (Google Play or iTunes).
Go to Stitcher.com
Click on the menu in the upper left hand corner and select “Get Premium”
Click on “Start Trial”
Select monthly
Add the promo code on the check out screen and tap ‘Apply’
Continue to Payment and follow the instructions on the screen