this is a nice plugin, however it has some issues in latest WordPress versions.
Using the following simple declaration from plugin examples doesn't work, the column is not added for users:
x_add_metadata_field('x_userField1', 'user', array( 'display_column' => true , 'description' => 'This is a field for a user. Enter information in here!' ));
If I change "user" to "post", it appears for the posts just fine.
Please get this fixed in next release.