Hi Pavan Kumar,
in my opinion you are trying to use GetActiveObject in a way that is in conflict with its specification. The quesion is not specific for MicroStation, but general for any OLE Automatiion development. I found pretty the same at StackOverflow (including some ideas).
My question is why you want to access MicroStation from external process, which is always more complicated and requires some extra effort in some situations, and don't create MicroStation add-in.
An regardless you acess MicroStation VBA/COM API from add-in or external process, you can use work file(s) to open and work with more design files. But it's hard to say if it's the right way without knowing more what do you want to achieve.
With regards,
Jan