I can’t get Steven to show up. Am I doing something wrong?

You need to obtain all 16 badges, thus beating the elite four in the process. Then go and defeat Red at the top of Mt. Silver. Then Steven should appear in the Silph Co building in Saffron City. Then pick the colour of the Hoenn starter you want (Green - Treeko, Blue - Mudkip, Red - Torchic). I'm purely guessing this is what you are looking for as you haven't made it very clear where you are in the story or what you have tried.