It is on the drivers side, on the inside of the frame rail near the back of the cab. It is yellow in colour and on the bottom of your fuel filter housing. The picture in the owners manual is looking up towards the housing if you were laying underneath the truck.
Dont worrie about draining it( unless you are running really bad farmer fuel lol)
I have never drained any of my Cummins filters. The only time I drain is when I'm doing a service, but that is only on the filter housing on the engine, there is a drain on that one that needs to be open to change the filter.
Just change the fuel/water separator out at service time and you will never have an issue.