What do you mean when you say you cant get the screen to work? Do you get nothing on the screen at all? No reaction from the screen when touched? No communication when reading/writing to the SSD? I have the exact same screen and have it up and running on an arduino mega. The libraries and examples work for me right out of the box without the need for modification.
As for the power requirements, I can run the screen and all of its advertised functions without the need for an external power supply. I simply plug a USB programming cable into the mega and into my laptop and everything works fine. Of course, for use outside of a dev environment, I use a 5V 1A power supply that happily runs the arduino and screen.