The parameters are x,y,n,meta1,meta2. x and y are in pixels, n is the entity type. A list of entity types can be found in various places (
here,
here, and the help in Ved). meta1 and meta2 depend on the entity, if not used, just set them to 0.
For example, createentity(8,8,3,0,0) creates a disappearing platform at 1 tile from the top left corner.