When you say it generates decimal numbers, do you mean that it generates 4.1, 6.2 etc whereas you want 4, 6?
If so, you can simply go into the variables and edit them to delete the bit after the decimal point. I know this is nowhere near as nice as a button that does it for you but it should work. I would think that a modification to only generates whole numbers (is that the right term), would be trivial.
Have you looked at Simple Calculated, it appears to me to do the same thing, only more simply. I assume there are some things it does not do that Calculated does, but I have not found out what yet.
I have found both question types to be a little "delicate", e.g. it doesn't do any checking at all on your answer formula so when a klutz like me forgets a brace it will fall over silently on preview , and I don't understand the outside limits messages that you have on your screen shot, as in I have experienced them myself and got rid of them by relatively random key pressing.