@ArunYoganandan - I think the reason you can’t control the Core right now is that it hasn’t yet been claimed. The claiming process is how you associate a specific Core with your account, and not anyone else’s. This process happens automatically if you configure the Core with your mobile application, but has to be done manually if you configure over serial.
In order to do this, follow the directions at the end of the “[Connect Over USB] (http://docs.spark.io/#/connect/connecting-your-core-connect-over-usb)” section of the documentation, and then continue following the instructions through the “[Claiming Your Core] (http://docs.spark.io/#/connect/claiming-your-core)” section.
If your Core then shows up in your web IDE, your Core has been claimed successfully, and you should be able to control it through Tinker and flash new code to it. Note that you may have to log out and log back into your Spark mobile app in order for it to recognize the new Core on your account.
Let me know if that works for you.