I Want A Nincada really bad ...
After beating the Elite 4 and obtaining the National Dex. You obtain the National Dex by getting every Pokemon Entry in the Sinnoh Pokedex as a View, not a capture. Visit Prof. Oak in Pal Park after doing these things.
Get national dex and go to professer rowan and professer oak would come and they give you pokeradar
1.Beat Elite 4
2.Battle all trainers in Sinnoh(this will help u see all Pokémon in Sinnoh)you only need to see Pokémon in Sinnoh no need to catch them :) hope it helped
This is easier compared to other games