National Dex

Well. It is first imperative that you seen all 150 Pokemon in the sinnoh dex. It can be a tall order. Commonly missed ones are unown, finneon, lumineon, dialge/palkia. Dawn is correct, once you see all 150 it should be fairly obvious that you go see the prof. Might I suggest lookin at your pokedex now, finding what numbesr you are missing, find out what Pokemon that is, and then find out where that one can be found.
I hope that hells

Wait, you are Nine years old? Wow, what a dedicated Pokemon trainer...and at such a young age too.

Anyway, Professor Rowan is not there because you have not completed the National Pokedex. In order to actually get the National Pokedex, you need to see all 150 Pokemon in the Sinnoh region. Once you do that, you will be able to get the National Pokedex. I have a hunch that you are missing a few. Find out what Pokemon you need, and come here for help.

I hope this helps.

You have to have seen all150 Pokemon on the Sinnoh dex first.A good way is to visit the walkthroughs.Some might tell what pokemona your missing and where to find it.

For the crap of lord,you need to finish the sinnoh dex,no need to capture them all you know.You also need to capture or kill Dialga or Palkia.And for the last time,your probably need to finish your main storyline first.