How to get Custom Fields working!?

I am using the ‘Fields Group’ plugin to create some extra fields for my website using the ‘iloveit’ theme. But can’t get the fields to show in the front end?

I have created the new fields within the plugin, then added this code to the post_item.php file where i want the fields to appear.

<p><?php $value = get_field(“post_submission”); ?></p>

But all I see on the front end is this error.

Fatal error: Call to undefined function get_field() in /home/content/p3pnexwpnas05_data01/72/2278972/html/wp-content/themes/iLoveit/post_item.php on line 677

What am I doing wrong?

Thanks for your help.