Step is literally speed. The direction is moves is based either on the Dir variable or if Angular is on, the Angle in radians. The engine will move it automatically. Problem is that assigning Angle is buggy in the current beta and won't work correctly.
Also, I mentioned this in zscript.txt but just in case it wasn't clear before, DeadSate must be -1 for the engine to move the projectile. If it is positive, it will not move and will count down each frame until 0 when it will disappear. If less than -1, it will not move at all period. When spawning a new weapon DeadState is set to -1 by default, so if you change DeadState to anything else, it can be screwy.