I'm trying to set up a Master/Slave program between 2 Uno's using NRF24L01. I must have missed something when I downloaded the NRF24L01 program.

C:\Users\User\Documents\Arduino\rx_v2\rx_v2.ino:2:18: fatal error: RF24.h: No such file or directory

include "RF24.h"

Where would I find this file?

You have not told us where you downloaded the library from? IIRC the TMRh20 version of the RF24 library has installation instructions and it is the version I recommend.

Have a look at this Simple nRF24L01+ Tutorial