Well, if you’re using a Mask as a spotlight-effect...? check out this Tutorial:
http://www.youtube.com/watch?v=RviA2M-GUms
Select the Circle Mask and open the Properties tab from the Inspector...
Move the Circle Mask in the Viewport and apply Position / Rotation / Scale
Keyframes as necessary...
Open the Keyframe Editor to make final adjustments if needed...
(click the Keyframe Icon, down in the far right corner of the timeline)
If you’re using a Spot-Light within a 3D Group...?
Select the Light, open the Properties tab in the Inspector...
Move the time-slider forward and keyframe the position of the Light accordingly...
Use the Properties tab or the HUD to keyframe Intensity, Falloff, Cone Angle etc...