Question 1: Where install it?
Slide it in \garrysmod\addons\
Question 2: The commands to know?
In the description of the product.
Question 3: Why buy it?
You are free to think that this is not an addon, but the real work is done, find a server that does not use music stolen. Find a gamemode that has its own mood music. It does not make much, for a creative original is one of them.