After creating mass floors, you can select each mass floor individually. When you move the cursor over a mass floor, the tooltip and status bar display the following:
Mass Floor : Mass Floor for <mass name> : <level>
If the status bar displays the name of the mass instead of the mass floor, press Tab to highlight the mass floor.
When the mass floor is highlighted, click to select it.
You cannot change the shape of a mass floor by manipulating it directly. Instead, change the shape of its mass. Revit automatically updates all affected mass floors and related information.