Sadly, after you catch Kyurem, you cannot go up those stairs after you catch Kyurem. Because when Kyurem hasn't been caught yet, it "creates" the snow for you to get up to it. And after Kyurem has been caught, it can't create the snow for you to get up there. But you can get up there by hacking. Hopefully this answers your question. Hope I helped!
Source: Experience
