Hi @shreyassule7738, welcome to the forum 
How are you making the connection? When I was adding a sensor previously, I found the mavlink connection had to be made with source_system=1 to communicate properly to the ground control software.
I have no experience with Mission Planner, and I don’t believe it has support for ArduSub. I would assume connecting is the same as for QGroundControl, but there may be differences I’m not aware of. If the above connection approach doesn’t help, you may wish to ask on the more general ArduPilot forum (this forum is targeted at marine robotics, including ArduSub, and Blue Robotics products/software)