How do you get the pokeradar cause I talked to prof Rowan and he gave me a national dex but didnt give What should do?

You will need to defeat Team Magma/Aqua at Mt. Chimney, then return to Littleroot Town to speak with your rival. After defeating your rival, return to Professor Rowan's lab, and he'll give you the National Pokedex and the Poké Radar device.