Make a channel in the CP at google.com/adsense and copy the required ad code .. Now login to the blogger CP and goto "Template" of your blog .. find the adsense code [if it already exists] with the new code with the channel information .. voila PS: you can save all this trouble by just adding a URL channel in your adsense CP .. That will track the impressions and click from the URL added .. No need to work with teh code ..
I think I couldn't do it cause google said I couldn't make a channel for a google owned domain. Maybe they changed it since then