The * is hiding under "Tables & Attributes"; choose the yellow box with * in it, then snap it to the end of the SELECT block, then snap the SELECT block on top of a FROM block which has a yellow "customers" block snapped into it. Voila, the "Show SQLCode" button now emits "select * from customers" (FF Beta, macOS)
I got there too after some puzzling. There's not even a distinction between attributes and table names to guide you. I got there by thinking "what would the correct syntax be". Kind of ironic.
