From Valve Developer Community
When you select an object, you're actually selecting controls in its animation set.
Note:You can't select objects that are part of the map you loaded because they are baked into that map.
To select controls:
- Expand the object's animation set in the Animation Set Editor.
- Click the control you want to select. Hold down and click to select multiple controls, or to deselect a control. Hold down and click to select a region of adjacent controls.
- Hover the mouse pointer in the viewport, hold down to show selectable controls, and click the control you want to select. Alternatively, you can lasso-select multiple controls by holding down and dragging. Hold down + and click to toggle the selection of controls. Hold down + and click to deselect controls in the viewport. Hold down + + and click to add to the current selection.
Note:In order to select controls in the viewport, you must be in either Motion Editor or Graph Editor mode.