Some early versions of the Model S do not include the power liftgate. Tesla does not offer a retrofit kit for this feature. The parts required include the liftgate linear motor, anti-pinch safety switches, the liftgate control module, chime, open/close switch and cinching latch.
There is a third party now offering a powered liftgate that can be installed in the Tesla for about $400 plus labor.
From a few that have installed it, it takes about 3 hours of work. When done, it can be controlled from the touch-screen, fob or the exterior rear liftgate button. It also includes a height memory feature similar to Tesla.
The one limitation is it does not have pinch sensors like the Tesla design, but will detect an obstruction and reverse direction.
This Powered Liftgate DIY thread at TMC may prove helpful too.