From Blue Mars Developer Guidebook
Creating Clouds in the City Editor
1) RollupBar ->Entity ->Render ->Cloud position cloud to your preferred location.
2) Good habit tip: Once the cloud entity is placed a parameter list should be displayed right below Cloud1 should be a layers icon, change this to Clouds if not there make a layer Clouds
3) Still in RollupBar under Entity go to MTL add your material through the Material Editor try looking at Materials ->AR ->Common ->Clouds ->pick one of these that you like. (once you selected your material go to Assign Material to Selection this is icon(sphere pointing to the cube) is at the top of the Material Editor) the material should now be in the MTL box.
4) Below MTL, Entity Params, Material Layers, is Entity Properties: under CloudFile go to game/libs/clouds/camera_clouds/pick one of these or make your own.
5) For movement go to Entity Properties under movement select AutoMove to True adjust Speed with Speed it will give you direction too.
6) To scale cloud use Select and Scale.