Can't remember who posted what but...
I believe Zorua is event only, and you get Zoroark which you breed for Zorua (though i might misread that article). TYhe others like Celebi, you either have to wait for an event, last year was Victini event, recently there has been Mewtwo and Arceus events, and i remember a year ago was Shaymin and Darkrai events for Platinum. Check Serebii website for current and upcoming events (left hand list bar thing). Or if you have them on a Generaition 4 game, you can trade them over (one way, they cannot return to Gen 4), via the Trade Station thing, east of Nimbasa, once you defeat Unova Elite Four. Keldeo has not been released yet, in fact despite us all knowing it existed, only a few weeks ago did Nintendo publicly announce Keldeo and Genesect....fudge knows why
Hidden Abilites, those through the Dream World and Zen Mode Darmanitan and that lv50 Musharna, they will only pass down abilities if they are female. and you cannot breed the female with ditto, it has to be same egg type
Pokemon Dream World, is accessed through the Pokemon Global Link, you put in your details after registering for an account. You can register one black and one white game maximum, and i don't think you can change them once you've registered (i'm not sure if restarting your game gives you same or different code, so be warned). You put a Pkmon to sleep via the Game Sync function of the C-Gear, and go to the website. (You need to have wireless connection set up for your DS). It is accessible once per day, and you gain Dream Points by winning mini games, logging on, making Dream Pals, watering others gardens. If you win a mini game, you befriend it, and if you skip the tree of dreams a few times, you can visit the island (where pokemon and items are found) multiple times, though more you visit on that day, less pokemon there are. you can take one pokemon a day back to your game, and any others you befriend vanish forever. the website allows you to be logged in for an hour, so make sure your done before it logs you out or you might not be able to send pokemon to the games. berries are found, and you can grow them, and they can be sent to your games, or swapped with others via the Share Shelf. there are 5 levels, some are unlocked from the beginning, others require you to have a certain number of gym badges, or a certain number of dream points. it can take a while to unlock, maybe a few weeks, but you can track your dream point total in the profile page where yu log in.
I have a question actually, how do you tell if a Pokemon has been hacked? I'bve just started trading, nothing out of the ordinary so far, but I no idea how to check if they are hacked or cloned or whatever. I am only trading so i can breed one for myself, but from sounds of it, hacks leave a marker on the game somehow?