Hi, as Goeff pointed out, the data are stored in ~/.ros/rtabmap.db by default. When clicking on Download Map or Graph under rtabmap_ros/MapCloud rviz plugin, the map is download in RVIZ to be visualized. Data is not saved somewhere. If you want to save the 3D point cloud to a file (PCD, PLY), the easiest and most flexible way is to open the database with rtabmap standalone application, then do File->Export 3D clouds...
\$ rtabmap ~/.ros/rtabmap.db