How to join Google affiliate network

Hello,
I have tried to sign in to Google affiliate network with my Google Ad sense account email id but did not go through. I would like to be in Google affiliate network so as I can put advertisers & show their ads on my site.
When I try to add an affiliate it says: Affiliate Network .
Sorry, your Google Account is not registered with this product. Sign in as a different user.
So what could I do now?
Thank you.
