Well, this seems obvious but since you've asked; first download the pieces from the DLS, and then go to Boat's website. I understand he has a tutorial there.
you can either email me at: TrainzFan@Aol.Com
or you can private message me
OR
if you want hands on learning on how to work the atls
try me on skype and i can show you exactly how to work things
if you would like
-Taylor
Search 'ATLS' on the DLS for the assets. Note that most ATLS assets are Trainz Build 2.4, (TRS 2004) but they work in all Trainz versions up to and including TS 12.