If you don't have a mobile website
In order to use AdSense for mobile, you must have a mobile-compliant webpage; adding AdSense for mobile ad code to a standard webpage will not display ads.
If you don't currently have a mobile website (and chances are, if you're not sure, then you don't have one), you can create one by rewriting your existing website in a mobile markup language, such as XHTML, WML, or CHTML. You'll also need to make sure the layout of your website is properly formatted to display on mobile phones.
In addition, your mobile website will need to be developed using server-side scripting such as PHP or ASP. Many web hosting providers support PHP, and in many cases, you can PHP-enable your webpages simply by changing the file extension from .htm to .php. Thereafter you can paste the PHP code snippet into the section of the page where you want the AdSense for mobile ads to appear.
If you are considering building a mobile website, we recommend that you focus on building a site for users of high-end devices.
For more information on building a mobile website, check out GoMo: An Initiative From Google.
Once you've created a mobile webpage that complies with our program policies, you're welcome to add Google ads to it!