Well here is what you should do:
Go to Beta Blogger template page, click on 'Edit HTML' tab and tick the checkbox of 'Expand Widget Templates'. Look for the following lines of Beta Blogger template HTML code usually found somewhere after the half-way mark of the entire codings:
Then place your Google Adsense code after:
Now after doing so, the problem you'll faced is that Blogger will not accept the codes thus unable to save changes made or the Adsense ad will not appear correctly or at all when checking on your blog. The issue is with the following Adsense codes:
There are 2 solutions to this, which is either to remove those 2 codes completely or manipulate the codes. Removal would be totally against Google Adsense TOS so that leaves manipulation as the only obvious choice.
So here are the solutions:
1] Replace with //-->
Now upon viewing the source codes of your blog from the browser (right click on mouse and select 'View Source in Explorer' or 'View Page Source in Firefox'), you will notice that the Adsense codes will appear exactly as it should be thus problem solved.
The blog which I learnt this from is BloggerDigest and the originator who started whole thing which we're all very thankful for is by Louiss from BloggingSecret.
On this note I would highly recommend those of you who like myself manages several other blogs to use the Channels feature in Google Adsense to track click-through performance and hits on each ad placement or blog.