Select an object > Motion panel > Parameters > PRS Parameters rollout
The PRS Parameters rollout provides tools for creating and deleting keys. PRS stands for the three basic transform controllers: Position, Rotation, and Scale.
To create a PRS transform key:

Create Key/Delete Key—Creates or deletes a move, rotate, or scale key at the current frame. These buttons become active or inactive depending on the existence of a key type at the current frame.
For example, if you're on a frame containing a Scale key, the Scale button is inactive in the Create column because a key already exists. At the same time, the Position and Rotation buttons are inactive in the Delete column because there are no keys of that type to delete.
Position/Rotation/Scale—Determines the contents of the Key Info rollouts that appear below the PRS Parameters rollout on the Motion panel.