BuddyPress.org

You only need two functions now, there was some fixes added recently to make this easier.

function my_custom_activity_meta_stuff( $content, $user_id, $activity_id ) {
	bp_activity_update_meta( $activity_id, 'bpcat', $_POST['bpcat'] );
}
add_action( 'bp_activity_posted_update', 'my_custom_activity_meta_stuff', 10, 3 );

function add_bpcat_form_func(){
  $output ='
	<div id="bpcat">
		<select name="bpcat" id="bpcat">
		  <option value="" disabled selected>Choose a Category</option>
		  <option value="update">Update</option>
		  <option value="news">News</option>
		  <option value="video">Video</option>
		  <option value="image">Photo</option>
		  <option value="advertisement">Advertisement</option>
		</select>
	</div>
';

  echo $output;
}
add_action('bp_activity_post_form_options', 'add_bpcat_form_func');