This tutorial was meant for a child theme. I can’t tell if you copied the entire functions.php file to your child theme or if you’re editing the original Twenty Twelve theme.
If you have not created a child theme, you can download a blank child theme at the bottom of this article, http://zeaks.org/create-a-wordpress-child-theme it contains an empty function.php file.
Copy content.php from your twentytwelve theme to the child theme folder.
Edit the child theme templates instead.
Once you do that, link your function.php file.
There’s no need to copy the entire contents of functions.php to the child theme functions file and it will cause problems if you do as the functions are already in the parent theme.
You can also download the child theme with the code already added at the bottom of the tutorial.
Yes, I have download the theme twenty twelve excerpts and uploaded it. But still i can’t see the thumbnail..please check my blog…any issue with featured tick or not i have also see one check box excerpts in the screen options. I have also used the ajax plugin…but no positive results..