Sign up for the beta. If you get approved, Log in, click the AdSense for content tab and under Ad Type, select AdSense for feeds unit.
http://services.google.com/ads_inquiry/aff

i try to register my feed, but got such answer

Thank you for your interest in AdSense for feeds.

Unfortunately, we're unable to accept your application for the program at
this time. Because AdSense for feeds is currently in beta, we're unable to
accept all applicants into the program. If we're able to extend our
service to you in the future, we'll be sure to let you know.

You'll just have to wait.. ;)