In a genesis powered site if you want to add a small piece of content just after the main post content you need to add the following code in functions.php file:

add_action( 'genesis_after_entry_content', 'wbxpress_after_entry_content' );
function wbxpress_after_entry_content() {
	echo 'A piece of extra content just after the main content visible to everywhere';
}

The above piece of extra content will be visible everywhere i.e. in the home page, archive page etc. But if you want to display it only in single post then add a conditional statement as follows:-

add_action( 'genesis_after_entry_content', 'wbxpress_after_entry_content' );
function wbxpress_after_entry_content() {
	if (is_single())
	echo 'A piece of extra content just after the main single post content';
}

If you also want to display in single post as well as page then the code will be:

add_action( 'genesis_after_entry_content', 'wbxpress_after_entry_content' );
function wbxpress_after_entry_content() {
	if (is_singlular())
	echo 'A piece of extra content just after the main post/page content';
}

Please refer conditional statements for any other situation.