If your question is how do you USE the Pokemon Radar, just to to your Key Items, select it, and select 'use' in grass, and not on your bike.
If your question is how do you GET the POkemon Radar, then what you do is see all of the Pokemon Sinnoh has to offer and pester Professer Rowan. He'll give it to you.
right after you get national dex
Route 224 Poke Radar
If you use the Poke Radar you will have a chance of getting a shiny Pokemon. But the actual Pokemon will not USUALLY change by using the Poke Radar.
you must evolve shelgon, who evolves from bagon. He is on route 210 with the poke radar
The correct spelling is Poke Radar instead of PokeRadar. Poke Radar is a key item in the Pokemon game. Its function is to point out where you can find Pokemon in grass, including some rare Pokemon that are only available when you use the Poke Radar.
To get the Poke Radar you gotta fill up the Sinnoh Pokedex, talk to the professor, and he will give you the National Pokedex and the Poke Radar.
right after you get national dex
The Poke Radar is not available in Pokemon Sapphire. The Poke Radar is an item exclusive to Pokemon Diamond, Pearl, Platinum, X and Y.
you can get bagon on Pokemon platinum on route 210 using poke radar
Route 224 Poke Radar
Route 212 with the poke Radar
You can find it at route 228 with the poke radar
use poke radar on route 210
Mt.cornet use a poke radar (it is rare)
route 210 (use poke radar)
Route 210 South (Poke-Radar)
On route 229 with the poke radar(rare)