please note that you should subscribe to kitti topic "kitti/velo/pointcloud",
you can play the rosbag file kitti_2011_09_26_drive_0002_synced.bag to find the "kitti/velo/pointcloud".
to convert kitti datasets to rosbags you can use kitti2bag
example : rosbag play -l kitti_2011_09_26_drive_0002_synced.bag
for more informations about the statistical outlier removal filter go to http://pointclouds.org/documentation/tutorials/statistical_outlier.php
for more informations about the voxel grid downsampler go to http://pointclouds.org/documentation/tutorials/voxel_grid.php