Currently there is a Keldeo distribution live via Nintendo WFC. This download is available anywhere where you can connect your DS to a Wifi Connection. This distribution is only for Black 2 and White 2 (the last one was BW only). The distribution will end Feb 12, so make sure you get it while you can!
As a side note, if you take Keldeo and the other Swords of Justice (Cobalion, Terrakion, and Virizion) to Pledge Grove behind Alder's House in Floccesy you can teach it Secret Sword. Once Keldeo learns Secret Sword in BW2 it was change into its Resolute Forme.