Or, just somewhere in the track header, or empty track space, or on the Insert > Video Envelopes menu. Opacity envelopes and Fade to Color envelopes are track envelopes; Velocity envelopes are event envelopes.
///d@
Allow me to point out that in the Insert / Video Envelopes menu, the item is named "Track Composite Level". If you're looking for "Opacity", you won't find it. ;)