The straight-to-Lavos teleporter gate should be available immediately upon being granted access to the square. The "flickering" graphic effect, which is fairly small to begin with, only happens about every five seconds. It isn't a constant shimmering, like what is often shown with key items or interaction points. It can be rather easy to overlook if one doesn't know precisely where to lock one's gaze.
At any rate, walking to the back wall of the right-hand teleporter pad and pressing the "Interact" button (default is A) should open the gate and start the final boss battle. If it doesn't, there's something amiss.
Are you certain you chose New Game + from the main menu and not the standard New Game? Double check Crono's stats before you attempt to open the gate, and make sure his level reflects the fact that the game has been played through once before; that it's
not in the 1-3 range.
@ forum: Are there any circumstances in NG+ in which that gate would
not be accessible? Every time I've played NG+, it's been there, right from the start.
