According to Blogger and Google’s AdSense, if you placed the AdSense within 48 hours or less, it may take up to that long for the AdSense "bot" to start crawling your blog and your ads to start being displayed on your blog.

If it has been more than 48 hours and you have placed the AdSense code on your blog correctly, there may be a few reasons your Google AdSense has not appeared yet. For example, there may not be enough content on your blog for Google to find the most relevant ads to display, or their "bot" simply needs more time to find your Website or your website may be considered adult-oriented or offensive.

For more detailed reasons why AdSense does not appear yet on your blog, visit this Google help link:

https://www.google.com/support/adsense/bin/answer.py?answer=43998&ctx=en:search&query=adsense+not+yet+displayed+on+blog&topic=0&type=f