To use the built in cheats, click on the "Solving" option in the menu at the top of the game window. You can then check if a letter or word is correct by clicking on that letter or word, and then selecting either "Check Word" or "Check Letter". If you are really stuck and want the game to solve the puzzle for you, you can also click on "Complete Puzzle."

To use the additional cheats such as reveal letter or suggest word, you must first switch to smart mode. To do this, click on "Options" and then choose "Smart Mode". Once you have done this, all of the cheats should be available in the solve menu.