Assuming you’ve added the column correctly and not hacked it in somehow, then the manage_${post_type}_posts_custom_column
filter should do what you want. The example in the Codex is about the best I’ve got given the sparcity of information in the question:
add_action( 'manage_posts_custom_column' , 'custom_columns', 10, 2 );
function custom_columns( $column, $post_id ) {
switch ( $column ) {
case 'book_author' :
$terms = get_the_term_list( $post_id , 'book_author' , '' , ',' , '' );
if ( is_string( $terms ) )
echo $terms;
else
_e( 'Unable to get author(s)', 'your_text_domain' );
break;
case 'publisher' :
echo get_post_meta( $post_id , 'publisher' , true );
break;
}
}
Note that the second parameter passed through the filter is your post ID.