From the documentation it looks like the pointcloud_to_laserscan_node takes it input on the cloud_in topic, so you should remap that to the pointcloud output topic from your velodyne.
rosrun pointcloud_to_laserscan pointcloud_to_laserscan_node cloud_in:=/your_velodyne_points

(replace /your_velodyne_points with the pointcloud topic from your velodyne)