To lay a simple one:
Lay two sections of track and leave a gap between.
Choose a one track level crossing from the "Objects" tab and place it on the baseboard.
Adjust the rotation to suit the track direction.
Now, go back to the track tab and choose the "move" tool.
Grab the end of one section of track and pull it to the track end on the level crossing - once the two circles match up, the track will "smap" into place. Do the same with the other piece of track.
Now, go back to your Obkects tab and choose "Splines".
Look for a Road that you like the appearance of.
Lay two sections of road, either side of the level crossing. Once laid, traffic should start flowing.
To tidy up, use the level tool (looks like a red and white ruler) to bring the level of the terrain up or down to match the road and the railway track. (You may need to adjust several times before you get the eddect you want!)
Now, once you have the hang of it, start messing about with double track and quad track level crossings.
Once you're happy with all of that, search the forum for posts on the ATLS system written by a guy by the name of "boat". The "ATLS" system allows you to build a complete controlled roadway system with traffic behaving properly at both traffic lights and level crossings. You should get the hand of constructing basic level crossings first though!
Hope that helps!
Ian