yes you do have to beat the game to get to giant chasm. the reason being is you dont get the national pokedex until you beat the game. the eastern part of the unova reigon has pokemon from past reigons and you cant encounter pokemon from past generations if you have the national pokedex. ditto is a past generation pokemon.
Giant Chasm
You can catch them at the Giant Chasm, the open area before Kyurem's cave.
Go to the Giant Chasm's Plains (after you go through the cave you will end up here) and by the way its a 15 percent rarity (normal grass).
You can catch Kyurem in Pokémon White in the Giant Chasm.
go to the giant chasm and the metangs there learn it
Giant chasm
Giant Chasm
Ditto is in the giant chasm ounce you beat the game
Go to the Giant Chasm and walk in the grass for awhile
You can get Ditto in the Giant Chasm after you get the National Pokedex.
After you beat the elite four you can catch it in the giant chasm there is also a legendary Pokemon in the giant chasm called kyurem.
The plains at Giant Chasm. Encounter rate= 15%
in white you find ditto at giant chasm in the plains (well crater forset) and you have a 15% chance of encountering it.
You get ditto when you go to the Giant Chasm they are found at level 50 and up there that is also where you get Kyurem.
Near the cave where you find Kyurem (Giant Chasm) in the Double Grass.
You can catch Ditto in the wild at The Great Chasm.
I believe it is found in the Giant Chasm area. If not, check out serebii.net, they've got good information on the game there.