How can I copture keldeo?

I don't think those locations have been released yet,for both of the Pokemon that you are looking for, most likely they might be part of a event just like past legendarys like shaymin and Darkri, get a certain item or download something and you can get the Pokemon. Like I said as of right now, I don't believe you can get them any way unless there is a action replay code that could be made, but not sure about that either.

Yes, Keldeo, Meoletta, and Genesect are the only three Pokemon that haven't officially been released by nintendo yet. Keldeo can only be obtained through a future nintendo event when you have the musketeer trio in your team(Cobalion, Terrakion, and Virizion), and then he appears. He can also learn secret sword, a move that can be taught through a move tutor. Meoletta can learn Relic song, also through a move tutor, but it isn't related to the musketeers of Unova, and is also a future event pokemon. Genesect is a Bug/Steel legendary that can be obtained through talking to someone in team plasma next to a factory ownedby them, and you can give him certain drives to change his signature move, techno blast. Unless you hack or use some AR code, ect, it's not possible to get them by other means than just waiting.

You can not catch meloetta unless you have cheats, hacks,or gliches.same thing with genesect and keldeo.
This is the only way I know how to get it and buy wating

Dont use cheats they will mess up your game like the walk through wall chaet

To capture Keldeo, Meloetta, or Genesect, you will have to use an Action Replay device. You can buy one off of the internet and use the codes that we provide for Pokemon White on this website.

I hope this helps.