Help on adsense

HI ,

I have applied for adsense a couple of days back. Now they have asked me to place the ad code on the site before activating the account. I have created two ad units and placed the code in the advertising section in header and right side bar. But there are no blank ad unit as they told me . ??. Also in the admin section there is an adsense tracking code field . What is that and where can I find the code?

KIndly guide me what to do as this is my first attempt for adsense.

Regards,