I don't know if there is a code. But you can complete de Dex by seeing all Pokemon in platinum. All Sinnoh-dex Pokemon can be found ingame.?
it's immpossable to get the national dex without seeing all 150 sinnoh Pokemon (this doesn't include manaphy, which is just extra if u see it)
You can catch them on Iron Island once you have the National Pokedex. You get this by seeing all the Pokemon on the Sinnoh Pokedex.
You get the sinnoh one basically after starting the game, the you get the national pokedex when you´ve completed the sinnoh pokedex (by seeing all the Pokemon in sinnoh)
By seeing all of the Pokemon there are in Sinnoh.
The secret garden.. To access this you must complete the Sinnoh Pokedex by seeing all Sinnoh Region's Pokemon.. Then if you want to catch Shaymin you must have the event key item Oak's Letter...
To get your second (national) pokedex you have to complete the first (sinnoh) pokedex by seeing all the Pokemon in sinnoh. when you have, go to the professors lab in sandgem town and show him the pokedex, then professor oak will come and give you the national pokedex.
It's easy. Complete your Sinnoh Pokedex by seeing all 150 Pokemon that can be found in Sinnoh. If you've battled every Trainer on the mainland along with the Elite Four you should be fine.
First you have to complete your first one (sinnoh dex) by seeing all the Pokemon in sinnoh. Next go to professor Rowans house and show him your pokedex.Then professor Oak will come and after a long chat he will give you the national pokedex.
By seeing all the Pokemon in sinnoh and going to rowen's lab.
Go to snowpoint city after beating the Pokemon league and speak to the sailor next to the boat and he asks you if you want to go to the battle zone and then he takes you there but to get further you have to complete the sinnoh pokedex by seeing every Pokemon in sinnoh and then the people blocking route 225 and 230.
by seeing all 150 sinnoh Pokemon including palkia.