He is in Sandgem Town, in the Pokemon Lab after you see all the Sinnoh Region Pokemon.
TO GET SHAYMEN IN LAND FORME
Yes. He was the young Prof. Oak when they travelled back in time.
you have to see all of the Pokemon and then you get the national pokedex from Prof. Birch and Prof. Oak in Prof. Birch's lab after completeing the Sinnoh pokedex.
In the national Pokedex its Wartortle and you only get it by exchange (for instance: HG/SS speak to Prof. Oak after beating Red) in Sinnoh Pokedex its Prinplup.Either you fight a rich kid in front of the Mansion where a book tells you about Manaphy or you choose Piplup from Prof. Rowan as a starter.
go talk to prof rowan in eterna city 50 times then talk to prf oak 50 time and you will get it!
Prof Birch is NOT in Diamond Prof OAK is in Eterna City.
Go to Prof. Oak.
TO GET SHAYMEN IN LAND FORME
YOU DON'T, you only find professer Rowan.
If you have the National Pokedex, Prof. Oak will be in a house in southern Eterna City.
You can give it to a Porygon and trade it so it will evolve into Porygon2.
for one its Prof. Rowan and they are Piplup, Turtwig and Chimchar! hope i helped.
complete a normal pokedex then go to palpark and talk to prof oak
For the shinno pokedex, it is given to you from Prof. Rowan in Sandgem town at his lab. For the national pokedex, it is given to you from Prof. Rowan and Prof. Oak in Prof. Rowan's lab after completeing the Sinnoh pokedex.
What to do after you talk to prof oak about groudon
you have to do prof. oak in bed then he gives you the Oak mail you have to battle prof. oak then he gives you the Oak mail
After completing the sinnoh dex, go to Prof. Rowan and Oak will arrive.He will give you the National Dex.