Place the pressure plate on the ground, open it for edit and in the side bar of the item properties, under "Scripts" open the "PUZZLE_PressurePlate" script and type a name for "EventOn" and "EventOff". For example "plateStepOn" and "plateStepOff".
Then you can make a script in the story editor with something like:
IF
CharacterItemEvent(_Player,_PressurePlateGUID,"plateStepOn")
THEN
ShowNotification("Stepped on plate");
<whatever you want to do>
For a lever you can do something like the above with the "PUZZLE_Lever" script. Or do a script like (and place a PointTrigger where you want the poison cloud to appear)
IF
CharacterUsedItem(_Player,_LeverGUID)
THEN
CreateSurface(_PointTriggerGUID,"SurfacePoisonCloud", 5.0, 10.0);
ShowNotification("Created poison cloud");