Just to be more explicit, did you do the following steps?
1. Install the Arduino IDE (and set the board type to Arduino 2560 -
can't remember the exact board name - it's the only one with 2560) -
not sure that you really have to do this, but it can't hurt for
later...
2. Connect printer and computer via USB,
3. Look for a new device in System Manager (assuming Windows here),
4. Install driver (find the right driver in the drivers subdirectory
of wherever you installed the Arduino IDE - I'm pretty sure that I had
Windows find the right one, but "Arduino MEGA 2560.inf" should work, I
believe),
5. Go back to System Manager and see if there's a new COM port - if
not, the driver didn't install, go back, correct. If so, note the COM
port #,
6. Install Pronterface/Slic3r,
7. Set the COM port in Pronterface to the one that you found
8. Try one of the jog dials to see if you can control the printer (use
a 1mm dial at first, in case something's funky)
It'd be helpful to know how far you got in the above list. I hope that
it's accurate, going off memory here.
SC