How do I get a hoenn dex in this game?

The pokedex your using is the Hoen Dex.

Level 30 my friend

Catch a nincanda, then leave an extra two spaces in your party Pokemon, evolve nicanda to ninjask at level 20, then after that look at your party Pokemon and poof theres shedinja.

I guess but any ball could catch those poke, I use a pokeball to catch both of them

Lol you start out with the Hoenn Dex.
Corphish evolves at lvl 30
Have a Nincada at level 19, a rare candy, at least one Pokeball, and five Pokemon in your party. Then use rare candy on your level 19 Nincada, then it will evolve into Ninjask. If you've done this right, when you go to check your party, a Shedinja should be in the sixth spot.

4. Rayquaza can be caught with an ultra ball so save it for deoxys.

The Pokedex is the Hoenn Dex.
Corphish evolves at Lv:30.
Get Nincada to Lv:20 with 5 or less Pokemon with you and a Pokeball in your bag.
Save the Masterball for whenever you must use it.
And Deoxys can't be caught in Ruby. You must trade with FR, LG, or Emerald.

Your game is already set to the Hoenn Dex; Corphish evolves at level 30; Shedinja appears in an empty slot in your party when Nincada evolves; the Master Ball is best left for Latios.

You have an hoenn dex at the beginning of the game, professor birch gives you one

Corphish evolves at lvl 30

To get shedinja, get a nincada, make sure you have at least one empty spot in your party, and evolve it. After you evolve it, there will be a ninjask and a shedinja in your party.

Save your masterball for latios, but if not, deoxys

I can't help you on the corepish and the shidinja thing but ypu get the hoenn pokedex from prof birch in the beginning of the game but if you mean the thing that has the map and stuff in it you get that at rustburo after you help the devon corp dude you go to the devon corp building and talk to the old dude.

1)You should have gotten your Hoenn Pokedex at the beginning of the game! Look at the top thing on your menu when you hit start-right above "Pokemon" should be "Pokedex". If you mean the National Pokedex, you get it after you become champ.
2)Corphish-Evolves at level 30 into Crawdaunt.
3)Shedinja-First you need a Nincada, when you evolve it at Level 20 be sure you have room in your party (5 or less Pokemon) and a Pokeball in your bag; when Ninca evolves into Ninjask, it'll give you Shedinja, too.
4)Masterball-If you think you can get to Deoxys??? Not without cheating or a Aurora ticket from Nintendo you can't! And if you can cheat, then you can get more masterballs....so, I don't know. Try getting him without it, if you're unsuccessful too many times, use it, because I don't think there's any other Pokemon you'd need it for more if you aren't cheating, but what if you find an awesome Shiny somewhere where you might loose it? It's up to you, but I'd use Ultraballs then Timerballs for a while before giving up that Masterball.

You start out with a Hoen dex, I would say use your master ball on Deoxys.

You start out with the hoen dex, use masterball on Deoxys, and they both evolkve on level 30.