You can creat you Adsense when you reach the Gold Level. You need to fill up the Google Adsense application which will show the status as "Pending" because it takes time for Google to approve which is purely on the basis of your active performance in KSP. Depending upon your active performance in KSP, Google will approve your application. It may take 1 week, 15 days or sometimes even a month or more. The more you are active in your contributions to this website, the chances of approval are more.