For the purposes of this explanation, I'll use BNSF50 as an example of a Grade Crossing, becuase hes the only creator I know of off the top of my head.
So, open Object Menu in Surveyor, Then Open the Find Asset Search box at the top. In that box type "ATLS". Grab a Slave, a Controller, and a Traffic Stopper of your choice, and drop them down anywhere near where the Road and Track run over each other.
Open Spline Object Menu, create two spline points in the road, 1 each side of the Tracks, and delete the new middle piece. Grab the remaining Ends, and attach them to the Long Sides of the Traffic stopper, and place the Stopper in the correct orientation and height over the Track.
Next, grab one of BNSF50's Grade Crossings, and put it somewhere around the Crossing itself, somewhere that makes sense. In theory, you could put it anywhere, but that would defeat the purpose. You want it over the road, just in front of where it crosses the Rails. Usually you want it just a little closer to the rails then where the spline point for the Traffic Stopper is. Drop a second Grade Crossing Device on the other side and do the same.
Now, open the Track menu. Pick ANY track that says "Invisible" in it. Grab it, and string it from the Pointy end of the Slave, to the first Grade Crossing Device. From the other spline point on that device, string more over to the Traffic Stopper, and then again from the other side of the Traffic stopper to the last Crossing Device. As long as all 4 of these assets are connected with invisible track, the order doesn't really matter, just don't make a junction anywhere along its track path.
While you're in the Track menu, go into the Track Object Sub Menu (Top Middle Tab of the three Tabs at the top of the Track Menu), and grab "ATLS Trigger". Place one each on the Normal Track (Not invisible track), on either side of the road. Grab the "?" tool on the bottom right of that menu, and open them up, and set them each to "Channel 1"
Exit the Track Menu and open the Object Menu. Grab the "?" and select the Controller, set that to Channel 1, and then do the same with Slave.
That should be the basics needed to set up your first ATLS Grade Crossing. Plop a Locomotive down AWAY from the Crossing, Quick Drive it over the Crossing and see if it goes off. There are TONS more options and assets you can use, but these are the basic ones needed.
-Falcus