The following game aide is very easy to operate. Every turn BEFORE rolling the dice you click either "Light" or "Heavy" for the corresponding Catan version. After pressing the button an effect is displayed that will have an impact to the game. All effects only count for 1 turn. There is more to it than just random effects and some effects will only happen based on what has happened before so don't refresh the page in the middle of a gaming session! However, you can freely switch between "Light" and "Heavy" in the course of one game but just be sure that all the other players agree with this decision! "Light" selection effects are generally just economical and "Heavy" will have the same effects but also a chance of losing or gaining buildings.
If your version of Catan is not available try and use the "Catan, base game" software since this one is more "all round". If you desperately wish to have your version available in this web app then you can donate the game to the maker of this software and he will add the game for you!