I did it with my MacBook Pro trackpad, and a finger on the control button. That works fine for the entire game, but Simon drives me nuts! Using a mouse does not seem to make the matter easier, I just got to get a good sequence. I even abort sequences I see are going to be hard. And it never helps when in the middle of it Li and Naija start hugging. Get a room already!
One night I worked on him for 15 minutes until I had enough, and the next morning did it first try. I think that may be a method: The Simon Says game uses parts of our brain that get little use in the present world. Try the game, give your brain a night to process the new task, and its easier. Other times I did it in one sitting, after a zillion tries. I think it also helps if after you fail, you wait 10 seconds or so for the old pattern to flush out of your short term memory, so as to not confuse you when you try the new one.