The widget should have the same slug (name) on both your dev site and in your new plugin.
Here is some test generated code I exported.
if( function_exists('register_field_group') ):
register_field_group(array (
'key' => 'group_5421d12d3b08c',
'title' => 'User Registration',
'fields' => array (
array (
'key' => 'field_5421d133e4826',
'label' => 'File Upload',
'name' => 'file_upload',
'prefix' => '',
'type' => 'file',
'instructions' => 'Upload a file',
'required' => 1,
'conditional_logic' => 0,
'return_format' => 'array',
'library' => 'uploadedTo',
),
),
'location' => array (
array (
array (
'param' => 'widget',
'operator' => '==',
'value' => 'recent-comments',
),
),
),
'menu_order' => 0,
'position' => 'normal',
'style' => 'default',
'label_placement' => 'top',
'instruction_placement' => 'label',
'hide_on_screen' => '',
));
endif;
As you can see, this add a “File Upload” to the “recent-comments” widget. recent-comments is the slug associated with that widget. This location value must match the widget you are trying to target. Make sure you are assigning a slug when you generate the widget. See the codex info on creating widgets for more info.
