Hi, Your resistor values will be fine for 5v output R1 120r and R2 360r 1/2watt resistors will be fine.
Now here is the problem, as you said you are inputing 15v and wish to draw a maximum of 2A. Because the output voltage is 5v you are wasting 10v this has to go somewhere.
10v x 2a = 20watts of power, this power needs to be dissipated through a large heat sink!
Looking at the data sheet of the device it has a thermal resistance junction to ambient of 35c/w. Which means for every 1 watt of power the LM338 will be 35c + the room temperature. This is with out a heat sink so in your case 35x20=700c+room temp. The LM338 would cut out long before it gets anywhere near this temp.
So you will need a large heat sink or a computer cpu cooler.
Other options would be to reduce the current draw or reduce the input voltage.
To understand more about thermal resistance and heat sinks watch this video.
Hope this helps
Don
A quick calculation shows that the following heat sink should work
**broken link removed** resulting in an heat sink temp of approx +80c for 20watts dissipation.