how do u get the national dex in pokemon pearl?

Question

btw what is a pokeradar?

Answer 1

you have to see all the 150 pokemon in the Sinnoh region. If you fought everyone you saw, you won't have a problem doing it.
You gotta beat the elite 4. Then professor oak will upgrade your pokedex to the national dex.

Pokeradar - is a device that allows you to look for rare pokemon on grassy areas. Some pokemon will only appear with the pokeradar. Also, it lets you see the location of shiny pokemons, very rare though.

Answer 2

you get the national dex from professor oak after you've seen all 150 pokemon (you can get dialga's info from cynthia's grandmother after catching palkia) and the pokeradar is a key item you get with the national dex, and you use it by standing in the grass and when using it, it wil lmake different patches of grass shake, showing you where pokemon are, they will also sparkle a little bit, and heres what they each mean

green- probably common, may be uncommon
yellow- probably uncommon, may be rare
white- probably rare, may be shiny (different colored)
starry- shiny

Answer 3

you need to see all the 150 pokemons and pass the pokemon leaugue.