oneTarek on "[Plugin: Advanced Custom Fields] Fatal Error: Call to undefined function the_field()"

Though this plugin is enable on my site , it shows a fatal error where I call the function the_field()
Fatal error: Call to undefined function the_field() in /home/..../wp-content/themes/.../page.php

Is there any solution?

Thanks in advance

http://wordpress.org/extend/plugins/advanced-custom-fields/