I am using PlantUML API to generate Activity Diagram.
I have used GraphViz before and used rank parameter to define layout
rank=LR [for left to right]
I am trying to find similiar thing in PlantUML where I can change its orientation to "Left to Right" instead of existing layout i.e. "Top to Bottom".
No, you cannot with current version (7876). You can use the following command with State, Class, Component or Usecase diagrams:
(see http://plantuml.sourceforge.net/usecase.html#Rankdir)
However, this feature will be added in next version.