How can I let google scholar find conference name of an arXiv paper?
Google scholar crawls the web automatically and indexes all the papers that it finds. Your paper will not be listed with the conference name before it is actually published. And since the conference has not taken place in your case, it will not be listed as such (and how should google know that the ArXiV paper and the official versions are exactly the same - which they probably aren't?).

So you have to wait until the official version of the paper appeared (e.g., on the publisher's web page). You can then tell Google Scholar to merge the two papers into one in your account.