I can't figure out how to create a new sign-on for the Sudoku on my Kindle Fire. Love the game. It would be nice if the undo was simpler. It should only have to click once to undo the last action so you can do many undo levels quickly. It would be nice if it would save the a game at the current level so you can return to that choice that you made if it turns out that it was incorrect. Sometimes you have no alternative but to guess and if you do and you want to return to that state to make the other choice, there is no way. I sometimes take a picture on my camera and return to that layout, but it would be easier if they just had a button.
To create / change your credentials go Settings and at the bottom enter the Email and the Password you wish to use.
As a side note, making a guess goes against the very essence of Sudoku, which is why we decided against "bookmarking" feature. However, we are considering it for inclusion in a future release. Stay tuned.