I have recently downloaded the Rstudio and I created a R Notebook. However the "Viewer" doesn't show me the Notebook when I save it (in the instructions says it works). I have seen the same issue in other forums, but with no answers.
I had the same doubt, but i found it here: Tools -> Global options -> R markdown -> Show output preview in, then select Viewer Pane. Next time you click preview (not save) it'll be showed where you want.