If you want the ad to display in the middle of all posts (old and new), you will have to hack your blog’s html and add some codes to it. I stumbled on the trick here, implemented it and works fine on my blog. I even use the trick to make Adsense ads appear in the middle of posts on Blogger mobile view.
This script in the trick actually looks for number of
(line break) tags inside your post and display ad unit.
How To Get Started
==> BACK UP your blog template. Very Important!
==> Generate your Adsense ad code from www.google.com/adsense
==> Parse the Adsense ad code at www.freehtmlparser.blogspot.com (paste and click “Encode”)
==> Login to your Blogger dashboard and back up your template ( Very Important! )
==> Go to “Template” > “Edit HTML“
==> Use CTRL F to search your HTML for the JQuery script below:
If it exists, move on to the next step. If not, copy the script above and paste directly above < / body > in your blog’s html ==> Use CTRL F to find