Not getting 5V output
In the comments you mentioned that you used I_adj = 3 A. Check the electrical characteristics on page 4 or 5. That current is at most 0.1 mA. I suggest reading the datasheet in more detail to get a better understanding of how the circuit operates and to be able to calculate the proper value of R_2.

EDIT: To give some more information to the answer... the integrated circuit will force 1.25 V accross R_1. The current flowing through it equals 1.25 V / R_1. The current coming from pin ADJ is supplied by the datasheet and is at most 0.1 mA. From Kirchhoff's current law, you can tell that the current flowing through R_2 is the sum of these two currents. The output voltage equals the sum of the voltage drops accross both resistors. We know that the drop accross the first resistor is 1.25 V, and the drop accross the second one is current*resistance. Knowing the current, you can calculate the output voltage from the resistance. Or you can express the resistance from the equation and calculate that from the output voltage.