Instructions:
1- Open the attached file with notepad++
2- Replace line 53 with your Responsive Ad code
3- save as index.php
ftp to your blog and rename the existing index.php (inside theme folder) to indexbkp.php
drop your new index.php which you saved in step 3.
This will show Ad after first post, screenshot:
use <br> for spacing above and bottom of the ad code as per desired.
Download link below: