does the effect of kneeling trigger when she is marshalled? or does she have to be in the table and then other Rhllor characters trigger the effect?

Melisandre can trigger her reaction when she herself is deployed. You did just deploy a R'hllor card, you are in the window for reacting to the "after you deploy a R'hllor card" triggering condition and, since she is in play, her reaction is actionable.

It's really no different than something like Littlefinger ("Reaction: After you marshal Littlefinger, draw 2 cards."), except that she is referring to herself by trait instead of by title.