Yeah, I will agree it is a "hard" game. To be honest, even I have a hard time solving the last level on purpose. In theory you can solve the last level just like the 4th level, but about 60% of the time you end up with 2 panes in a parity flip that I have yet to determine how to solve. I actually came up with an even more intense version of the last level that would require you to solve not only the correct panes and cubes, but their orientations to one another as well. Still on my "todo" list, so maybe eventually! I also would like to port it to PC, I think it would work well on a much larger display. Hope everyone is having fun with it!
I purchased this app a little while ago. It is fun and challenging. The tesseract rendering is great and the rotation is very responsive. I hope the developer considers making a "twisty puzzle" simulator app!
I was in the same position too, never having used a code before. I don't have the phone at hand to give you a better guide, but in the App Store app there is a button in the first screen, at the bottom, that lets you redeem it.