Call your provider for the code or pay for one online. If you screwed your phone up trying the "free" method too many times, you might need to USB unlock. Search for more information about this.
SGH-I337 (AT&T) 32GB
P-ROM (1st Gen.), De-Odexed, AMDL
Auto-LOKI OUDHS CWM
Make sure you read and understand the "PINNED" threads in the "General" and "Q & A" sections of the AT&T S4 forum BEFORE you ask for help. Use the forum's "search" function to find posts with additional information. You should not expect others to help you if you haven't tried to help yourself first. Also, read, understand, and follow the "XDA" rules.