Before applying, ensure that your site has the following criterias
1) Have high-quality content and traffic
2) Have a reasonable volume of visitors already using the website
3) Receive majority of traffic from United States / Canada / United Kingdom
4) Have English as the primary language
5) Abide by there Program Guidelines
After fulfilling all this you can proceed to register for Media.net Publisher Network
by visiting there website http://media.net
and clicking request an Invite
After that you will recieve a mail stating
Thank you for your interest in Yahoo! Bing Network Contextual Ads, powered by Media.net.
This email confirms that we have successfully received your request for an invitation to our web publisher program.
(1) We will begin issuing a limited number of accounts daily.Due to the extremely high volume of interest that we have received, we currently have a backlog, and hence it may take us up to 2 weeks to get back to you on the status of your invitation request.
(2) Each website that you have submitted is individually reviewed by us for approval. There are several factors including advertiser demand, that determine which websites get approved and the order in which we process invitation requests. We will do our best to get back to you as soon as possible.
We truly appreciate your interest in displaying our ads on your website(s).
We are excited to begin working with you!