It's bolted to the right-hand end of the block as you look at the engine from the front. It has an electrical connection and two high pressure fuel lines attached to it. The purpose of the requlator is to lower the pressure of the fuel system during the warm-up period of the engine. This reduces the pressure acting on the top of the airflow meter thus allowing it to rise easier, this in turn richens the fuel/air mixture.
Think of it as a kind of choke mechanism for an injection system.
Hope this helps
Toby