"So I have to make an input (SlotRounds) be used in a for loop..."
Who says you "have to" do anything, specifically, about how to implement the instructions? The answer to that statement is "No you don't!" :)
As for the Q? "is there a better way?", sure...make the control variables what they are -- the controls, NOT the working variables.
Don't hardcode stuff like this; can't ever change anything without completely rewriting the code.
nRounds=input('How many rounds to play?');
NB: the logic test for winnings is not going to work as you wish; read carefully on how Matlab does if tests on arrays. Also, despite that, you seem to have the likelihoods in wrong sense; a whole lot of people are going to win big in your casino--you're likely not going to be able to stay open long! :)