You dont go to a pokecenter you go to the trasfer lab between the bridge and black city there in the lab you will need two ds talk to the man at the end with the second ds have in a gen4 game and go on ds download play then you choose 6 pokemon to transfer -you cant transfer back you play a little mini game where there are loads of bushes and you have a sling shot and pokeballs the pokemon you picked will jump in and out of the bushes -if you dont catch them in the time limit (i think its 2 minets ) the ones you dont catch will be returned to your gen4 game it will then ask if you want to transfer more yes or no then thats it i think -the pokemon will appear in your box
to get the pokemon to jump, hit the bush while they stick their heads out (the pokemon stays in the same bush untill it jumps out). If you get a chance to put them to sleep, aim at the Z's, not the bush
( requires two people of course ) You go upstairs at the poke center and talk to middle guy and you should know the rest.
P. S. they might tell you or something ; D