How to use mulebuy codes effectively (complete guide)

Quick question for the community - I'm trying to use mulebuy's codes feature but running into some issues. I've followed the guides I found but something isn't clicking. Specifically, I'm trying to order jeans and the process seems different from what others have described. Has anyone else encountered this? The interface looks different from the screenshots I've seen. I've already tried clearing my cache and using a different browser. Same result. Any suggestions would be really appreciated. I've been stuck on this for 34 hours now. Thanks in advance!